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