21. As tourist, you will need to use underground in London, which can be very convenient.
A. a; a B. a; the C. the; an D. the; the
22. I don't think we can take those pollution problems any longer, as they are threatening our life now.
A. on hand B. by chance C. for granted D. for example
23. I made this myself but it was who taught me.
A. him B. by him C. himself D. he
