问题如下：
The next day, Harding instructs Yellow to revisit their research on BYYP, Inc. Yellow’s research leads her to believe that its shares are undervalued. She shares her research with Harding, and at 10 a.m. he instructs her to buy 120,000 shares when the price is $40.00 using a limit order of $42.00.
The buy-side trader releases the order for market execution when the price is $40.50. The only fee is a commission of $0.02 per share. By the end of the trading day, 90,000 shares of the order had been purchased, and BYYP closes at $42.50. The trade was executed at an average price of $41.42. Details about the executed trades are presented in Exhibit 1.
Based on Exhibit 1, the opportunity cost for purchasing the 90,000 shares of BYYP is:
选项：
A.$22,500.
$60,000.
$75,000.
解释：
C is correct.
Opportunity cost is based on the number of shares left unexecuted in the order and reflects the cost of not being able to execute all shares at the decision price. The opportunity cost is calculated as:
Opportunity cost = = (120,000 – 90,000) × ($42.50 – $40.00)
= $75,000
老师，您好！题干中这句话，我的理解不是很清晰： he instructs her to buy 120,000 shares when the price is $40.00 using a limit order of $42.00.此处的decision price 是40，但为何limit order是42呢？如果此处42是他决定购买的价格，为何不是decision price呢？谢谢
