题目：Alan bought a futures contract on a commodity on the New York Commodity Exchange on June 1. The futures price was USD 500 per unit and the contract size was 100 units per contract. Alan set up a margin account with initial margin of USD 2,000 per contract and maintenance margin of USD 1,000 per contract. The futures price of the commodity varied as shown below. What was the balance in Alan’s margin account at end of June 5?

A.     USD -1,120

B.     USD 0

C.     USD 880

D.     USD 1,710

解答为：

我的疑问是，4号的时候维持保证金还有1750，为什么第五天亏损才290元就要再补仓到初始保证金？