题干

对于定义域为D的函数y=f(x)和常数c,若对任意正实数ξ,∃x∈D使得0<|f(x)﹣c|<ξ恒成立,则称函数y=f(x)为“敛c函数”.现给出如下函数:
①f(x)=x(x∈Z)
②fx=
1
2
x
+1x∈Z
③f(x)=log2x
④fx=
x
-
1
x
.其中为“敛1函数”的有(  )

A:①②

B:③④

C:②③④

D:①②③

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答案(点此获取答案解析)

C

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