下列运算正确的是（　　）

A．a⁴•a²=a⁶

B．5a²b-3a²b=2

C．（-a³）²=a⁵

D．（3ab²）³=9a³b⁶

It is no use trying to talk him into studying hard; he can only be         by force.

A．upset

B．harmed

C．rescued

D．persuaded