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. -3II. 1III. 5
Ionly
IIonly
Iand II only
IIand III only
I,II, and III
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为什么不能向前推呢?求解答。
你向前推的话就相当于假设了对任意数t,有t-2在数集中,而题目中没有。
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