17. You don’t know about the difficulty I had ______ the work then at all.
　　A. done           B. to do           C. for doing      D. in doing

　　18. ______ is well-known, the environment in China is badly in need of improvement.
　　A. It           B. That       C. As          D. What

　　19. Charles Babbage is generally considered ______ the first computers.
　　A. to invent        B. inventing         C. to have invented      D. having invented

　　20. With oil prices keeping ______, people are hesitating whether to buy a car or not.
　　A. rising        B. arising      C. raising        D. arousing