In the 1930s the two countries were at war and his father was fighting at front.A. the

问题补充：

In the 1930s, the two countries were at war, and his father was fighting at front.

A. the; 不填

B. 不填; the

C. the; the

D. 不填;不填

答案：

A. the; 不填

(答案→)B. 不填; the

C. the; the

D. 不填;不填

解析：at war 处于战争状态是固定搭配，at the front 在前线，结合句意，故选B