<span>The correct answer is C. Swimming is great cardiovascular exercise.
In this sentence, "swimming" is a gerund. It functions as the subject of the sentence. In the first option, "swimming" is part of the verb phrase ("were swimming"). In the second option, "swimming" is part of the adjective and describes the team. Only in the final sentence does "swimming" function as a noun, or a gerund.</span>
A gerund is a verb that ends with the <em>ing</em> form (or the present participle form), however, it does not act as a verb but as a noun. In the sentence "Swimming" is acting as a noun (It refers to the skill of swimming or the sport of swimming itself) and it's the subject as well as it is what is being dealt with in the sentence.
