Mr. Smith gradually a knowledge of the subject.
A. attained
B. achieved
C. required
D. acquired
网友回答
A. attained
B. achieved
C. required
(答案→)D. acquired
解析：acquired （=gain for oneself by skill or ability, by one's own efforts or behavior）（由技术、能力、努力或行为）获得; 得到：He has acquired a good mand of English language.（他已精通英语。）
achieve, vt. 取得（胜利、成功等）, 实现（目标、目的等）。
attain, vt. 达到（目的等）, 取得（成就等）：1） I hope you will attain your object.（我希望你会达到你的目的。）2）He attained success through hard work.