I did not understand what the lecturer was saying because I had not read his book.

• A. 
  What the lecturer wrote and said was too difficult for me to understand.
• B. 
  The lecturer's book which I had not read was difficult to understand.
• C. 
  I found it very difficult to understand what the lecturer said in his book.
• D. 
  I would have understood what the lecturer was saying if I had read his book.