According to a certain type of rate, a shipper cannot be charged less than theappropriate rate of 4kg in most cases, even though his particular consignment mayweigh less than this, we call this type of rate （）.

Accordingtoacertaintypeofrate,ashippercannotbechargedlessthantheappropriaterateof4kginmostcases,eventhoughhisparticularconsignmentmayweighlessthanthis,wecallthistypeofrate（）.

A、bulkunitizationrates
B、minimumcharges
C、classrates
D、specialcommodityrates