GWD1-10

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

我选的是C，可是答案是D，不明白啊！

留意问句: MUST

-3只是可能出现,而非必然:如果-1是该数集的最小值时,则-3不在该集合内.