In 1972, appellant pleaded guilty to two counts of armed robbery,1 and one count of assault with intent to commit rape while armed.2 He was sentenced on two counts to two fifteen-year terms under § 5010(c) of the Federal Youth Corrections Act, 18 U.S.C. §§ 5005 et seq. (hereinafter, FYCA). On the third count, the trial court imposed an adult sentence of ten to thirty years, with execution suspended, and placed appellant on adult probation for five years.
Appellant was paroled from his FYCA sentences in February 1975. After appellant was tried and convicted of a robbery committed while he was under the supervision of correctional authorities, the trial court held a show cause hearing to decide whether appellant’s probation should be revoked. Pursuant to Super.Ct.Cr.R. 35, appellant filed a Motion to Correct an Illegal Sentence, requesting that the adult sentence be vacated and replaced by an FYCA commitment. Although the government conceded the illegality of the original sentence, the trial court denied appellant’s motion, revoked his probation, and ordered execution of the adult sentence.
The trial court’s written opinion noted that its conclusion at the time of the original sentencing was that appellant would benefit from an FYCA sentence. However, the court also reiterated its opinion that imposition of the adult sentence was the only method of guaranteeing adequate supervision of appellant should he violate an eventual FYCA parole. The court indicated its dissatisfaction with the parole board’s “woefully inadequate” monitoring process and its “generous” and “irresponsible” decisions with respect to rearrested felons.
Although we think the trial court sought a commendable objective, we must hold that the method used was a legal impossibility. Under 18 U.S.C. § 5010, if the trial court decides that the youthful offender3 will benefit from FYCA treatment, it has three alternatives: probation under § 5010(a), or commitment under either § 5010(b) or § 5010(c). On the other hand, an adult sentence pursuant to § 5010(d) may be imposed only if the court determines that the offender will not benefit from FYCA treatment.4 Here, the trial court made an explicit finding that appellant would benefit from FYCA treatment. Accordingly, it consistently could not impose a valid adult sentence, because that requires a finding of no benefit. An offender cannot be sentenced both under the FYCA and as an adult; the two alternatives are mutually exclusive.
As to the possibilities for resentenc-ing on remand, it seems clear that once the trial court has found likely benefit for the accused and thus chosen to impose sentence under the FYCA, the only possible legal sentence on the third count is under the FYCA. The imposition of an illegal sentence cannot give the trial court the authority to do now what it could not have done originally.5
*653We note that the subsections of 18 U.S.C. § 5010 also are mutually exclusive. If the court finds that the youthful offender does not need commitment, it may utilize only subsection (a) to impose probation. Treatment under subsection (b) is exclusive of treatment under subsection (c), because commitment under the latter requires a finding that the offender will not derive maximum benefit from a commitment of less than six years. Thus, on remand, a sentence under subsection (c) is the only permissible sentence.
Accordingly, we reverse the denial of appellant’s motion, vacate his adult sentence, and remand to the trial court with instructions to resentence appellant under 18 U.S.C. § 5010(c).
Reversed and remanded.
. D.C.Code 1973, §§ 22-2901, -3202.
. D.C.Code 1973, §§ 22-501, -3202.
. A “youthful offender” is defined in 18 U.S.C. § 5006 as a person under the age of twenty-two at the time of conviction.
. United States v. Waters, 141 U.S.App.D.C. 289, 437 F.2d 722 (1970), required a trial court to make an explicit finding that the youthful offender would not benefit from FYCA treatment before it imposed an adult sentence. Accord, Dorszynski v. United States, 418 U.S. 424, 94 S.Ct. 3042, 41 L.Ed.2d 855 (1974) (decided after appellant was sentenced). See also Smith v. United States, D.C.App., 330 A.2d 519 (1974).
.The dissent cites four cases from federal appellate courts which stand for the proposition that when a trial court resentences an individual eligible for FYCA treatment, it may consider events occurring since the imposition of an invalid sentence. Jenkins v. United States, 555 F.2d 1188 (4th Cir. 1977); McCray v. United States, 542 F.2d 1246, 1247 (4th Cir. 1976); Rewak v. United States, 512 F.2d 1184, 1186 (9th Cir. 1975); United States v. Carmichael, 152 U.S.App.D.C. 197, 469 F.2d 937 (1972). In the first three cases, the resentencing was required because the trial courts imposed adult sentences without making the “no benefit” finding requisite under Dorszynski v. United States, supra note 4. In the fourth case, United States v. Carmichael, supra, the appellant was *653sentenced as an adult to five to fifteen years on a burglary count and three to ten years on each of two assault counts, the sentences to run concurrently. The court ruled that the evidence supported a conviction of only one assault, not two, and accordingly, set aside the second assault conviction, vacated the sentences and remanded for resentencing on the two remaining counts. We deem those cases inapposite to the issue here for in none of them was the appellant ever found likely to benefit from FYCA treatment, as was appellant.