The reason      he was late for school was      he had to send his mother to a hospital.

A．which; that

B．why; that

C．for which; because

D．what; why

抗体的多样性决定于

A．淋巴细胞的凋亡

B．淋巴细胞的种类

C．淋巴细胞的数量

D．淋巴细胞的来源

E．淋巴细胞的基因