While being vectored to the final approach course of an IFR appr

While being vectored to the final approach course of an IFR approach, when may the pilot descend to published altitudes?

A.Anytime the flight is on a published leg of an approach chart

B.When the flight is within the 10-mile ring of a published approach.

C.Only when approach control clears the flight for the approach.

正确答案是C