答案：D

题目分析：考查从句，首先分析句子，从句personally I doubt very much.缺少宾语，which引导的非限制定语从句

点评：本题考查的是个非限制性定语从句，增加了试题的难度，需要考生能够分析理解句子的结构，熟记连词关系词的句法功能。

即学即练：That art center is _______ used to be a factory, ______ millions of tractors were produced.

A. where; where    B. what; where     C. what; which         D. where; which

选B