单选题 Although most universities in the United States are on a semester system which offers classes in the fall and spring, some schools ______ a quarter system comprised of fall, winter, and summer quarters.
A. manipulate B. stipulate C. regulate D. observe

【正确答案】 D

【答案解析】[解析] 本题中，D项“observe遵守”符合题意，如：observe a rule(遵守规则)。其他三项“A项manipulate操作，利用；B项stipulate规定，保证；C项regulate管制，调节”都不正确。因此D项为正确答案。