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.

• A. It could be found by researchers that employees who have friends at the office are generally happier in their job.
• B. It is being found by researchers that employees who have friends at the office are generally happier in their job.
• C. It was found by researchers that employees who have friends at the office are generally happier in their job.
• D. It has been found by researchers that employees who have friends at the office are generally happier in their job.