27.

Answer and Explanation

Your Answer is

Correct Answer is C

Explanation

The subject of the sentence is Ballet Nacional de Cuba is recognized ... Therefore, the horizontal part cannot have a new subject-verb, and A/B excludes. According to the meaning of the sentence, it means "already used as a training place for world-class dancers", so having done [completed] has the correct form.

Item D: served as... is a non-predicate done, meaning passive, which means wrong.