In spring we can often hear some birds in our school.A. singingB. singC. to singD. sings

问题补充：

In spring, we can often hear some birds in our school.

A. singing

B. sing

C. to sing

D. sings

答案：

A. singing

(答案→)B. sing

C. to sing

D. sings

解析：句意：在春天，我们能经常听到一些鸟的鸣叫在我们学校。分析固定句型：hear sb./sth. doing 正在进行的动作；hear sb./sth. do表示经常听到。 故选B