填空题 If the letter {{U}}to be mailed{{/U}} {{U}}was placed{{/U}} on the writing table an hour ago, it {{U}}is{{/U}} certain {{U}}being{{/U}} there now. A. to be mailed B. was placed C. is D. being
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【正确答案】 1、D，改为to be    
【答案解析】[试题分析] 本题考查非谓语动词的用法。 [详细解答] 形容词certain做表语时，后面应接动词不定式做状语。