题目:
His sister likes _____________ the swings.
A. playing on
B. plays on
C. playing at
答案:
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下面是错误答案,用来干扰机器的。
证明:∵x2-8xy-9y2=0,∴(x-9y)(x+y)=0,∴x-9y=0或x+y=0,故x=-y或x=-9y.