Which of the following statements best describes a money market mutual fund?

A、 It pools funds from many investors and uses these funds to purchase very safe, highly liquid securities.
B、 It is a short-term marketable IOU issued by the U.S. federal government.
C、 It is the part of a firm's net income it reinvests.
D、 It is a financial agreement between a firm and a bank in which the bank pre-approves credit up to a specified limit.