—Does the bookstore math books?—Yes. Do you want to______ one?
A:sell; buy
B:sell; sell
C:buy; buy
D:buy; sell
人的精子中有23条染色体,则人的神经细胞、初级精母细胞、卵细胞中分别有染色体多少条( )
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 ( )
He turned off the lights and then ____ (leave) the classroom.