问题如下：
A trader contacts a broker to enter into a futures contract to sell 5,000 bushels of wheat for 600 cents per bushel. The initial margin is USD 3,000, and the maintenance margin is USD 2,000. Under what circumstance is the trader required to provide more margin? How much mar-gin is required? Under what circumstances can USD 500 be withdrawn from the margin account?
选项：
解释：
The trader is required to provide more margin if more than USD 1,000 is lost from the margin account. This will happen if the price of wheat falls by more than 20 cents because variation margin will then have reduced the margin balance by more than 5,000 * 20 cents = USD 1,000. The trader is required to bring the balance in the margin account up to the initial margin level of USD 3,000. USD 500 can be withdrawn from the margin account if the price rises by 10 cents or more (5,000 * 610 cents = USD 30,500, which is USD 500 more than the required initial margin of USD 30,000).
The trader is required to provide more margin if more than USD 1,000 is lost from the margin account.
This will happen if the price of wheat falls by more than 20 cents because variation margin will then have reduced the margin balance by more than 5,000 * 20 cents = USD 1,000.
——不是很理解，明明是持有futures to sell，是一个空头，为什么跌的时候还不挣钱。
而且wheat跌20cent，现货实际也只跌了1000，futures是保证金交易，保证金的变动应该远少于这1000才对。
就比如用股指期货交易，当合约亏很大时，期货的保证金变动是按比例算的；当保证金变动很大时，期货的合约变动就极其巨大了。
这道题不懂，盼复，谢谢。
