单项选择题
A.if s=4B.if s=4:C.if s==4D.if s==4:
A.a=[[1,2,3],[4,5],[7]]B.a=[1,2,3]+[4,5,6]+[7,8,9]C.a=[[1,2,3],[4,5,6],[7,8,9]]D.a=[[0]*4]*5
A.a[:10]B.a[1:]C.a[1:10]D.a[10]