The problem requires .
A. studying with great care
B. to study carefully
C. to be studied without carelessness
D. taking great care of studying it
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(答案→)A. studying with great care
B. to study carefully
C. to be studied without carelessness
D. taking great care of studying it
解析：本题考查动词用法。句中require doing = require to be done，表示要求做某事。根据句意选择A。句意：问题要求仔细地研究。