困惑 ttGWD 30-31

按常识来说，停车位应该有两个方向（车头朝内或朝外），那么此题是否应该选C呢？还是这种想法多虑了（题目没说有两个方向），直接选A就好？

Q31:

There are 5 cars to be displayed in 5 parking spaces with all the cars facing the same direction.  Of the 5 cars, 3 are red, 1 is blue, and 1 is yellow.  If the cars are identical except for color, how many different display arrangements of the 5 cars are possible?

A.    20

B.    25

C.    40

D.    60

E.    125

请教思路

My answer is C. There are two directions for parking of those cars.

第一步：三辆红车一模一样，连停的方向一样，5个位子选3个放红车，所以应用组合　C(5,3)

第二步：剩下一蓝一黄由于颜色不同，所以在剩下的2个位子全排列　P(2,2)

因为分两步走，所以用乘法: C(5,3)*P(2,2)=20

5!/(3!*1!*1!) = 20, 题中说了"all the cars facing the same direction",

可以看成是一个前提吧,就像求PROBABILITY那样.