The visiting professor gave us some advice, of great help to our English learning.
A. I think it is
B. I think which is
C. which I think is
D. which I think it is
网友回答
A. I think it is
B. I think which is
(答案→)C. which I think is
D. which I think it is
解析：句意：来访的教授给我们一些建议秒我认为这对我们的英语学习很有帮助。Which引导的是非限制性定语从句，在定语从句中which做主语，I think是插入语，所以选C。