A.30
B.40
C.50
D.20
E.15
[单选题]A.不能B.能C.无法判断
[单选题]A.0B.∞C.f(0,0)D.1
[单选题]A.B.C.D.E.
[单选题]A.B.C.D.
[判断题]A.对B.错