题目

下列各命题中,真命题是(    )A.若{an}成等差数列,则{|an|}也成等差数列B.若{|an|}成等差数列,则{an}也成等差数列C.若存在自然数n,使2an+1=an+an+2,则{an}是等差数列D.若{an}是等差数列,则对任意正整数n都有2an+1=an+an+2 答案：D

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