—What’s most useful invention in the 20th century? — computer I think.A. the A B. a A

问题补充：

—What’s most useful invention in the 20th century?

— computer, I think.

A. the, A

B. a, A

C. the, The

D. /, The

答案：

A. the, A

B. a, A

(答案→)C. the, The

D. /, The

解析：考察冠词。最高级前面要加冠词，表示特指。下文指的是上文提到在20世纪最有用的发明，为特指，用定冠词。故选C。