单项选择题

仪器的效率可用剩余度来衡量。剩余度R的计算式为______。
   A.;    
   B.;
   C.;
   D.

A.B.174.24.96:6088/Latex/latex.action?latex=bGdcZnJhY3sxMDB9e3hfezJ9LXhfezF9fQ%3D%3D'
C.D.174.24.96:6088/Latex/latex.action?latex=bGdcZnJhY3t4X3syfS14X3sxfX17MTAwfQ%3D%3D'
E.F.174.24.96:6088/Latex/latex.action?latex=bGdcZnJhY3t4X3syfS14X3sxfX17XHNpZ21hIFxzcXJ0ezJccGkgZX19'
G.H.174.24.96:6088/Latex/latex.action?latex=bGdcZnJhY3tcc2lnbWEgfXt4X3syfS14X3sxfX0%3D'