Lily plays basketball and ______________ to school. [ ]
A. go
B. goes
C. going
答案:B
设Sn是等差数列{an}的前n项和,且a1=1,a5=9,则S6=( ).
一般以( )作为评定材料切削加工性能的难易程度。
A.强度
B.刚度
C.弹性
D.硬度