请教一个题目 谢谢！

A owns 100 shrs of stock trading at 86 dolloar per share. and B is short 200 shares of the stock. A wants to buy 100 more shares if the price rise to 90USD, and B wants to cover his short position and take a profit if the price falls to 75 usd. the order that A and B should enter are:

limit buy @ 90 limit buy at 75

limit buy at 90 stop buy at 75

stop buy at 90 limit buy at 75

总是不理解 大家说说选啥 帮忙解释一下啊