使用猪瘟兔化弱毒苗（）天后产生免疫力。
A.1
B.4
C.8
D.20
参考答案：B
压力容器封头不包括（）封头。
A、凸形
B、锥形
C、平板
D、凹面
Directions: For this part, you are required to write a composition on the topic: Campus Life. You should write at least 120 words.
