[单选题]One of the puzzles still mystifying biologists is _______ when is the time for the organic to become old and finally die.(1分)
Ahow know cells
Bhow cells knowing
Chow cells know
Dhow do cells know
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参考答案
试题答案：
C
答案解释：
解题思路： 本题考查表语从句的用法。根据题意“一直让生物学家觉得是个谜的难题之一是：细胞如何知道生物体是什么时候开始老化，并最终死亡的”。句中“how cells know when is the time for the organic to become old and finally die”作表语，从句中主谓语词序不颠倒；D）选项不对；A)和B）选项有语法错误；正确答案为C）选项。
