A:充分不必要
B:必要不充分
C:充要
D:既不充分也不必要
——We are going on a trip to Shanghai next week.——_______.
用数学归纳法证明:1×2×3+2×3×4+...+n×(n+1)×(n+2)=n(n+1)(n+2)(n+3)4(n∈N*)