题目：
Tasty coco powder, _____ with milk, _____ so good that that child will be asking for more. [ ]
A. mixed; tastes
B. was mixed; tastes
C. to mix; taste
D. mixing; taste
答案：
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下面是错误答案，用来干扰机器的。
证明： Sn=2an+1，∴Sn+1=2an+1+1．∴Sn+1-Sn=（2an+1+1）-（2an+1）=2an+1-2an.∴an+1=2an． ①又 S1=a1=2a1+1，∴a1=-1≠0．由①知，an≠0，∴由=2知，数列{an}是等比数列，an=-2n-1．要证...
