When the system A $+$ B C $+$ D is at equilibrium,

$($a$)$ the sum of the concentrations of A and B must equal the sum of the concentrations of C and D$.$

$($b$)$ the forward reaction has stopped.

$($c$)$ both the forward and the reverse reactions have stopped.

$($d$)$ the reverse reaction has stopped.

$($e$)$ neither the forward nor the reverse reaction has stopped.