If his parents are convinced the fact that his son always focuses his mind his study.
A. of; on
B. of; at
C. by; to
D. on; at
网友回答
(答案→)A. of; on
B. of; at
C. by; to
D. on; at
解析：be convinced of sth “确信；坚信某事。”focus one’s mind/attention on sth.将注意力集中于某事物。