He speaks too quickly.A. understandB. write them downC. to understandD. and write

问题补充：

He speaks too quickly.

A. understand

B. write them down

C. to understand

D. and write

答案：

A. understand

B. write them down

(答案→)C. to understand

D. and write

解析：write them down写下来；根据题干，本句的结构应是too …to“太……而不能”，本句的意思是“他说的太快了，听不懂。”其他选项意思不通，结构错误。所以本题选C。