问题补充：
The expert offered us another piece of advice,of great help to our program.
A. I think it is
B. I think which is
C. which I think it is
D. which I think is
答案：
A. I think it is
B. I think which is
C. which I think it is
(答案→)D. which I think is
解析： I think 为插入语，干扰了定语从句结构的判断，从句中缺少主语，不能省略。故选D。
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