I think this is useful book.A. aB. anC. theD. /

问题补充：

I think this is useful book.

A. a

B. an

C. the

D. /

答案：

(答案→)A. a

B. an

C. the

D. /

解析：句意：我认为这是一部有用的书。Useful首字母的发音为辅音因素/ju/，所以前面用不定冠词a，故选A。