I think this is useful book so I will keep it for ever.A. aB. anC. /D. the

问题补充：

I think this is useful book, so I will keep it for ever.

A. a

B. an

C. /

D. the

答案：

(答案→)A. a

B. an

C. /

D. the

解析：由于useful 的第一个音/ju：/为半元音，所以用冠词a