16．函数f(x)={(2a−1)x+3a,x<1ax,x⩾1f(x)=\left\{{\left.\begin{array}{l}{(2a-1)x+3a,x<1}\\ {{a^x},x\geqslant 1}\end{array}\right.}\right.f(x)={(2a−1)x+3a,x<1ax,x⩾1，对于任意x1x_{1}x1，x2∈Rx_{2}\in Rx2∈R，当x1<x2x_{1}<x_{2}x1<x2时，都有f(x1)−f(x2)>0f(x_{1})-f(x_{2})>0f(x1)−f(x2)>0成立的必要不充分条件是((( )))