Tramps(ships that have no set times or routes)are ready at any time to （）any particular voyage.

A．订货时即支付货款；优惠待遇
B．订立合同；优惠条款
C．有顺序付款；优惠条款
D．订立合同；优惠待遇

A．shipping mark；port of discharge
B．place of delivery；shipping mark
C．port of loading；port of destination
D．port of discharge；port of loading