“每月的”的英语？

汽车司机每月的油量将限定为30升。Motorists will be rationed to thirty litres of petrol a month.每月的花费可能增加也可能减少，很难预测。Monthly costs can rise or fall unpredictably.他付每月的抵押借款利息有很大的困难.He's having a lot of trouble paying his mortgage ever...