If f(x)=x1/xf(x) = x^{1/x}f(x)=x1/x for x>0x > 0x>0, at what xxx value does f(x)f(x)f(x) have a horizontal tangent?
x=1x = 1x=1
x=ex = ex=e
x=1/ex = 1/ex=1/e
x=ex = \sqrt{e}x=e