应该选择b how long.这是一个带有宾语从句的句子,要选择的词是从句的引导词,a选项中的what既引导宾语从句,又必须在从句中作主语或宾语,而从句中本身有了主语和宾语,所以不能选.b和c选项都可以在宾语从句中作状语,但是根据句子的意思,应该选择b,这句话的意思是“我不知道他们一直通过写信保持联系有多久了.”
i have no idea _they have kept in touch with each other by w
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