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

Answer:

According to the key answers, “D” shall be the right answer, but I think it shall be C.

Anyone can explain?

对数集任意t,有t+2在数集当中。

从-1属于这个SET可以推出1、3、5、7……但是不能向前推，即不能说明对于任意t+2在set中，一定有t也在这个数集中