马上就要高考了,恳请英语高手帮忙解释一道语法填空题,

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马上就要高考了,恳请英语高手帮忙解释一道语法填空题,
如题：American like the motor car --every family has to have at least two ,one of (which)has to be a convertible(敞篷汽车）with 300 horse -power,Weinermans,another professor said.
【这是一篇高考模拟题中语法填空中的其中一个句子,该句中“which”为所填答案.请问为什么该句填空处要填“which”,而不用“that"或“them”?】

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