Which one of these statements about Conditional Routes(CDR) is t

Which one of these statements about Conditional Routes(CDR) is true?

A.The purpose of CDRs is to allow flights to be planned on ATS routes or portions thereof, which are not always available

B.Conditional Routes (CDRs) are permanent ATS route network.

C.CDR 1 routes are always available for any time

D.CDR 2 routes are only available at night

正确答案是A