明朝北京的紫禁城是指[ ]
A.京城
B.宫城
C.皇城
D.外城
答案:B
_______ is certain that practice makes perfect.
A.What
B.It
C.There
D.Anything
计算:
(-a2)3•a6=______;
(2a-b)(a+b)=______.