To my surprise, my good friend stay up to study to prepare for the next important exams organized by ten famous high schools.
A. should
B. may
C. must
D. can
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(答案→)A. should
B. may
C. must
D. can
解析：考查情态动词。should 应该，竟然，may 可以，must 必须，can 能，会，句意：使我吃惊的是，我的好朋友竟然熬夜学习为了准备下一次由十所著名高校组织的重要考试。所以选A。