John was walking to school he saw a cat in a tree.A. BecauseB. SinceC. WhileD. If

问题补充：

John was walking to school, he saw a cat in a tree.

A. Because

B. Since

C. While

D. If

答案：

A. Because

B. Since

(答案→)C. While

D. If

解析：A. Because因为 B. Since自从… C. While 当…时候 D. If假如；句意：当约翰在走路上学的路上，看见了树上有一只猫。题目中用了过去进行时，while多于进行时态连用而且多于表示延续性的动词连用，故选C