Do you think Maths useful subject? A. is aB. is anC. are aD. are an

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(答案→)A. is a
B. is an
C. are a
D. are an
解析：此题重点考查不定冠词的基本用法。冠词a用在辅音音素之前，an用在元音音素之前。单词useful的第一音素是辅音音素，故用a；又因maths是科目，做主语时谓语动词要用单数，故选A。