Their telephone number is 5675707. Have you     ?

A．written it down

B．written down it

C．written them down

D．written down them

污水排入河流以后，溶解氧曲线呈悬索状下垂，故称为氧垂曲线，氧垂曲线的临界点是指

A．污水排入河流后溶解氧浓度最低点

B．污水排入河流后有机物浓度最低点

C．污水排入河流后亏氧量增加最快的点

D．污水排入河流后溶解氧浓度为零的点