问题如下图：
选项：
A. 
B. 
C. 
D. 
解释：
Whione of the following statements is incorreregarng the margining of exchange-trafutures contracts? y tras anspretransactions require lower margin levels. If investor fails to posit variation margin in a timely manner, the positions mliquitethe carrying broker. Initimargin is the amount of money thmust positewhen a futures contrais opene A margin call will issueonly if the investor’s margin account balanbecomes negative. ANSWER: All the statements are correct, except If the margin account balanfalls below the maintenanmargin (not zero), a margin call will issue c，为什么不是存放posit呢？答案写的是open的时候，不是交易的时候呢
a应该怎么理解？？？
