A.
B.
C.
D.
[单选题]A.B.C.D.
[单选题]A.a=-1,b=1B.a=1,b=-1C.a=2,b=1D.a=2,b=2
[问答题]
[单选题]A.B.C.6D.5
[单选题]A.正三角形B.三角锥形C.正四面体形D.直线形
[单选题]A.必要条件B.充分条件C.充分必要条件D.既非充分又非必要