1.选B
be afraid of doing sth
be afraid to do sth
2.选A
这里的as可翻译成：作为
3.选A
watch/see/listen/hear sb doing sth
看见/听见某人正在做某事
watch/see/listen/hear sb do sth
看见/听见某人做某事
4.选B
one of后多用复数
如：one of + the +最高级+复数
Changjiang River is one of the longest rivers in China.