--- What useful book !
---Yes, it is. But I found difficult for me to read.
A. an, it was
B. a, it’s
C. an, it’s
D. a, it
网友回答
A. an, it was
B. a, it’s
C. an, it’s
(答案→)D. a, it
解析：英语中不定冠词的用法是根据后续单词的词首音素，如果词首音素时元音，则用an，否则用a。上文useful词首音素为辅音，故不定冠词用a。下文it做形式宾语指代后面内容，故选D。