问题补充：
I’m glad.
A. hear from you
B. to hear from you
C. hear a letter from you
D. to hear a letter from you
答案：
A. hear from you
(答案→)B. to hear from you
C. hear a letter from you
D. to hear a letter from you
解析：be glad to do sth，表示很乐意做某事，而收到某人的来信用hear from sb，故选答案为B
