21. 设函数f(x)={2−x,x⩽1log2x,x>1f(x)=\left\{\begin{array}{l}{{2}^{-x},x\leqslant 1}\\ {lo{g}_{2}x,x>1}\end{array}\right.f(x)={2−x,x⩽1log2x,x>1,则满足f(x)⩽2f(x)\leqslant 2f(x)⩽2的xxx的取值范围是((( )))