Grandpa is growing old. He keeps on the same thing over and over again.A. tellB. tellsC. t

问题补充：

Grandpa is growing old. He keeps on the same thing over and over again.

A. tell

B. tells

C. told

D. telling

答案：

A. tell

B. tells

C. told

(答案→)D. telling

解析：句意：爷爷在变老，他一遍又一遍地重复讲同一件事。keep on doing sth.持续做某事，后跟动名词，故选D。