---What do you think of the grammar book?---Oh, it really is _________ useful reference book, which is worth reading _________ second time．A.a; aB.an; theC.an; aD.the; the
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A解析a useful reference book表示“一本有用的参考书”，useful的第一个发音是［ju：］，属辅音开头，故用a; a second time表示“又一次”，作状语，故用不定冠词a。