It's dangerous to let the children who are go swimming in the river.A. too young toB. so

问题补充：

Its dangerous to let the children who are go swimming in the river.

A. too young to

B. so young

C. not old enough to

D. so old

答案：

A. too young to

(答案→)B. so young

C. not old enough to

D. so old

解析：若选A和C,意义不完整,因为这时let后面缺少了宾语补足语.