直接刺激B细胞产生抗体的是（）
A.抗原决定簇
B.胸腺依赖性抗原
C.胸腺非依赖性抗原
D.完全抗原
E.共同抗原
参考答案：C
____________ rivers can you see on the map? I can see only four.[ ]
A. What
B. How many
C. How old
D. Which
_____ of the students in this school is over 2000, _____ of them are girls. [ ]
A. The number; a number
B. A lot; some
C. A number of; many
D. The number; much
