单选题 Mr. Smith kept on (studying) Chinese and (use) it (until) he (had mastered) it.
A. studying B. use C. until D. had mastered

【正确答案】 B
【答案解析】[解析] 应改为using。and前后构成并列结构，都作为keep on的宾语，因此use也应和studying一样用-ing形式。