At that time the two countries were at war and his father was fighting at front
问题描述：
At that time the two countries were at war and his father was fighting at front
A.the;/ B./;the C.the;the D./;/
答
B./;the
be at war "在.交战中" 其中不用冠词,固定搭配.
at the front＝in the frontline "在前线"
at front"在某个位置的前面"
in front of "在.的前面/方"（甲不包含乙）
in the front of"在.的前部" 在...(内部的前面)（甲包含乙）
1.他在前线牺牲了.
He lost his life at the front.
2.我们应该支持在前线的弟兄们.
We should support the boys at the front.
3.He sits in front of the TV set.
他坐在电视机前面.
4.He sat in the front of the car.（他在汽车里）
他坐在汽车的前座.
5.He sat in front of the truck.（他在卡车外）
他坐在卡车的前方.
6.He sat in the front of the cinema.（他在电影院里面的前排）
他坐在电影院的前部.