Deposits provide much of the raw material for making loans and, thus, may represent the ultimate source of profits and growth for a depository institution. A deposit manager is trying to make the appropriate interest rate for deposits in order to balance the cost and profit. Among the following statements, which is correct?
选项：

A: The optimal interest rate should be determined when marginal cost rate equals to marginal revenue.
B: Margin cost rate decrease at first and increases as the rises of interest rate.
C: Higher interest rate should be taken therefore to appeal more deposits.
D: Amount of profit earned always decreases when interest rate increases.