Nowadays it’s very useful a foreign language.A. learnB. learnedC. to learnD. learns

问题补充：

Nowadays it’s very useful a foreign language.

A. learn

B. learned

C. to learn

D. learns

答案：

A. learn

B. learned

(答案→)C. to learn

D. learns

解析：句意：当今学习外语非常有用。强调句式， It is adj for sb to do sth，做某事对某人来说怎么样。本句中it是形式主语，指代后面不定式内容做形式主语，故选C。