For the answer to the question above, I believe it is various forms of be.
The most common linking verbs are, am, is, are, was, were, being, and been, so the correct answer is the third choice among the given questions.
Yep, its a metaphor........
Answer:
Since the middle of the 20th century, scientists <u>have found</u> cures for many diseases. However, so far no one <u>has discovered</u>
Explanation:
those are the perfect tenses for (find) and (discover)
