—Does the bookstore math books?—Yes. Do you want to______ one?
A:sell; buy
B:sell; sell
C:buy; buy
D:buy; sell
若数列{an}对于任意的正整数n满足:an>0且anan+1=n+1,则称数列{an}为“积增数列”.已知“积增数列”{an}中,a1=1,数列{an2+an+12}的前n项和为Sn,则对于任意的正整数n,有( )