下列计算正确的是（　　）

A．[2+（-2）]⁰=1

B．10^-4•10⁴=1

C．（10⁴）²=10¹⁶

D．（3×10）³=9×10³

--- Let’s go to play football, shall we?

--- Sorry, I can’t. I must go over the points the teacher explained ______ I forget them.

A．after

B．when

C．before

D．since