The little boy felt grateful the coach for his letting him the game. A. for / join inB.

问题补充：

The little boy felt grateful the coach for his letting him the game.

A. for / join in

B. to / to join in

C. with / join

D. to / join in

答案：

A. for / join in

B. to / to join in

C. with / join

(答案→)D. to / join in

解析：对某人表示感激可用feel grateful to sb，使役动词let后跟动词原形作补语，加入比赛或具体的活动用join in，故本题选D。