Set M contains numbers that satisfy the condition that, if integer x is in the set then x+3 will also be in the set M

题目

Set M contains numbers that satisfy the condition that, if integer x is in the set then x+3 will also be in the set M. If -4 is one of the value in the set, which of the following numbers must also be present in the set M.

I. -7

II. -1

III. 2

选项

I only

II only

I and II only

II and III only

I, II and III

解析

注意题目问的是“下列哪个数**一定**也在集合M中”，而不是“**可能**在集合中”。现在，如果-4在集合里，那么\(-4 + 3 = -1\) **一定**也在集合里；既然-1在集合里，那么\(-1 + 3 = 2\) **一定**也在集合里。-7有可能在集合里吗？当然，但我们无法确定它必然存在。答案：D

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