The basic equation of dynamic programming reverse order algorithm is ( )

A、fk(xk)=opt{vk(sk,xk)+fk+1(sk+1)}
B、fk(sk)=opt{vk(sk,xk)+fk+1(sk+1)}
C、fk(sk)=opt{vk+1(sk+1,xk+1)+fk+1(sk+1)}
D、fk+1(sk+1)=opt{vk(sk,xk)+fk(sk)}