21. She said she ______ that strange animal twice.
A. sees B. saw C. has seen D. had seen
22. ________ the machine begins to work, you can’t stop it _____.
A. While; soon B. For; right away C. Once; at once D. Until; immediately
23. Please don’t ______ sugar to Mr. Wang’s coffee. He prefers it with only milk.
A. mix up B. add C. mix D. add up
