A.12
B.16
C.21
D.25
E.30
[单选题]A.12B.16C.21D.25E.30
[单选题]A.B.C.D.
[单选题]A.必要条件B.充分条件C.充要条件D.以上都不对
[单选题]A.6B.7C.8D.9E.10