The old man is very healthy. I think he will live.A. to be 100 years oldB. 100 years oldC.

问题补充：

The old man is very healthy. I think he will live.

A. to be 100 years old

B. 100 years old

C. at 100 years old

D. in 100 years old

答案：

(答案→)A. to be 100 years old

B. 100 years old

C. at 100 years old

D. in 100 years old

解析：句意：这个老人非常健康。我认为他会活到100岁。动词不定式做状语表示具体的将要进行的动作，结合语境可知选A。