GWD MATH 求助

Q9:

A gardener is going to plant 2 red rosebushes and 2 white rosebushes.  If the gardener is to select each of the bushes at random, one at a time, and plant them in a row, what is the probability that the 2 rosebushes in the middle of the row will be the red rosebushes?

A.     1/12

B.      1/6

C.      1/5

D.     1/3

E.      1/2

Answer:

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Q10:

A set of numbers has the property that for any number t in the set, t + 2 is in the set.  If –1 is in the set, which of the following must also be in the set?

I.                    -3

II.                 1

III.               5

A.    I only

B.    II only

C.    I and II only

D.    II and III only

E.    I, II, and III

第一题 C(2,1)*C(2,1)/P(4,4)=1/6

第二题 只能向前推 不能像后推 选d

1/[C（2 3）+C（1 3）]=1/6

D

答案是这样的么？