— your pen friend Chinese now?
—Yes. He studies very hard.
A:Do; study
B:Does; studying
C:Are; studies
D:Is; studying
已知等差数列{an}的前n项和为Sn,a5=5,S5=15,则数列 {1anan+1} 的前100项和为( )
下列四种动物特征的描述正确的是( )