下列计算正确的是 [ ]
A．a·a3=a3
B．a2·a3=a6
C．（a2）3=a5
D．a5+a5=2a5
答案：D
I hadn't seen her for 10 years, but I ____ her at once.
A．realized
B．recognized
C．knew
D．found
两个力F1=4N，F2=6N，它们的合力不可能是（ ）
A．2N
B．10N
C．12N
D．8N
