问题
选择题
This kind of CD-ROM is very popular. Why not buy one before it___________?
A. comes out B is come out C. is sold out D. sells out
答案
答案：C
根据上下文的内容，本题选用的内容为sell out表示卖完的意思，相当于及物动词，可以有被动语态，故选答案为C
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