If the order of buying stock is ignored.

问题如下：

There are 20 stocks in the stock market. An investor wants to buy 7 of them for her portfolio. If the order of buying stock is ignored. Which of the following statements is the most correct?
 

选项：

The stock combinations are 6.

The stock combinations are 77,520.

The stock combinations are 390,700,800.

解释：

B is correct  

${l}C_{20}^7=\frac{20!}{(20-7)!7!}=77520\\$