问题
选择题
What _____ Kathy fell and hurt her knee? [ ]
A. weather
B. whether
C. through
D. if
答案
答案：D
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What _____ Kathy fell and hurt her knee? [ ]
A. weather
B. whether
C. through
D. if
答案：D
展开成x的幂级数，并求级数
的和．