Teachers be sensitive to the development level of each student so as to help them better.

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(答案→)A. need to
B. may
C. are able to
D. can
解析：考查情态动词。句意：教师需要对每一个学生的发展阶段很敏感，来更好地帮助他们。May可能；be able to能够；can可能，可以。need既可作情态动词，也可作实义动词。根据句意可知，A项正确。故选A。