卸货时发现包装或表面残损，船方签残后或最迟在提货前申请鉴定；易腐，易变质，易扩大损失的，需登轮了解情况，确定受损范围，判定致损原因。 ( )

34. When he knocked at the door, my mother was busy ____ supper.

A．preparing for

B．in preparing

C．to prepare for

D．to prepare