Had the doctor not been sent out at once the boy who was knocked down by a passing A B C

问题补充：

Had the doctor not been sent out at once, the boy who was knocked down by a passing

A B C

car would not have been saved.

DB been sent out ---been sent out for

答案：

(答案→)B been sent out ---been sent out for

解析：sent out for a doctor是固定搭配，意思是：去请医生。