Much to my surprise, I found his lecture on the civilization of Mesopotamia extremely interesting.

• A. Contrary to expectations, his lecture on ancient Mesopotamia was the most fascinating of all.
• B. I was fascinated by what he said in his lecture on the civilization of Mesopotamia though I hadn't expected to be.
• C. I hadn't expected him to lecture on the civilization of Mesopotamia, but he spoke remarkably well.
• D. It was at his lecture on the civilization of Mesopotamia that I realized how fascinating the subject is.