Which of the following is an example of collusion?

An employee includes a faked receipt in his expense claim, and the claim is signed by the employee's manager.

A vendor inflates the price of an item and remits a portion of the excess to the purchasing manager.

A vendor sends a duplicate invoice with a new invoice number, and the accounts payable system fails to detect the duplication.

An employee works with the IT manager to develop a program for identifying duplicate invoice payments.