---Listen! Who in the next room?-I think Sam is.A. singB. singsC. singingD. is singing

问题补充：

---Listen! Who in the next room?

---I think Sam is.

A. sing

B. sings

C. singing

D. is singing

答案：

A. sing

B. sings

C. singing

(答案→)D. is singing

解析：根据listen 可判断时态为现在进行时，故选D