Question

This is not a new theory; quite a lot of scientists ........... on it for several decades.
would have worked
had worked
have been working
would work
Other:
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Answer 1

Answer:-

• This is not theory; quite a lot of scientists had worked on it for several decades.

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Answer 2

Answer:

HAD WORKED

Explanation:

As the action was done in the past because the theory has been released now