2014 National Question 4 Single-choice Differentiating 2014_退役省自主命题 (2014·文) 4.下列函数为偶函数的是 A. $f(x)=x-1$B. $f(x)=x^{2}+x$C. $f(x)=2^{x}-2^{-x}$D. $f(x)=2^{x}+2^{-x}$ Answer keyD ☆ SaveCan't do it, show the explanation查看原卷 →