求助

miafy

25.
                3619-!-item-!-187;#058&003382

A fast-food company plans to build 4 new restaurants.  If there are 12 sites that satisfy the company's criteria for location of new restaurants, in how many different ways can the company select the 4 sites needed for the new restaurants if the order of selection does not matter?

 

(A)  48

(B)  288

(C)  495

(D)  990

(E)  11,880

答案是C,为什么不可以用12X11X10X9啊,

gwq

12个sites里面任意选四个，C12^4=12！/4！（12-4）！=495