Which of the Following Is an Example of Double Billing

Which of the following is an example of double billing?

A) A professor bills his own university, instead of the university he traveled to speak at for travel reimbursement.
B) A professor leaves his job at one university for another but continues to collect pay from that university while receiving pay from the second university as well.
C) A professor bills the same university twice for the same travel expenses.
D) A professor bills her university for travel expenses and an outside booking firm for her fee to give a speech at another university.