根据句意，汉语提示及首字母完成句子。

1. His favorite subject is social s_____.

2. My uncle knows about _____ ( 不同的) countries of the world.

3. She loves to _____ ( 旅行).

4. Li Lei is i_____ in Chinese.

5. The funny elephant is so i_____.

冬天某人入浴后突然晕倒，其原因是

A．全身血管收缩

B．血管容量增加

C．脑部缺血

D．血量减少