21. （ ）

(单选题)______ time went by, Herbert could finish all the math exerc (单选题)We Chinese have a dream —______ dream to turn a well-off lif (单选题)— May I look at the menu for a little while？— Of course, ___ (单选题)It worried her a bit ______ her hair was turning grey. (单选题)Most people work because it’s unavoidable. ______, there are (单选题)Don’t leave the water ______ while you are brushing your tee (单选题)— What’s the matter with you?— _____ the window, my finger w (单选题)Researchers in Italy say ______ drinking may length your lif (单选题)The manager was worried about the press conference his assis (单选题)Even the best writers sometimes find themselves ______ for w