Select the correct passive form of the given sentence.
Researchers have found that employees who have friends at the office are generally happier in their job.

It was found by researchers that employees who have friends at the office are generally happier in their job.

It could be found by researchers that employees who have friends at the office are generally happier in their job.

It is being found by researchers that employees who have friends at the office are generally happier in their job.

It has been found by researchers that employees who have friends at the office are generally happier in their job.