CSC chapter 9

The current quote on QRS common shares is $45 bid, $45.50 offered. Merna placed an order to buy 500 shares of QRS at the best available price. Which type of order has Merna placed?
A, Limit order; B, Market order; C, Stop-loss order; D, Good through order.
答案是B, 请问老师，为什么不是Limit order? 谢谢