This kind of exercise is most , and therefore is a good test of your students’ ability.
A. surprising
B. encouraging
C. exciting
D. challenging
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(答案→)A. surprising
B. encouraging
C. exciting
D. challenging
解析：考查形容词的用法：A. surprising惊人的，B. encouraging令人鼓励的C. exciting令人兴奋的 D. challenging有挑战的，句意是：这种练习是最有挑战的，因此是检测学生能力的很好的办法。选A。