Federal law caps the total number of tons of so2 emissions that the epa can sell permits for and lowers the cap periodically ; the laws of supply and demand determine the price at which permits for one ton trade 联邦法规限定环保署出售二氧化硫排放许可的总吨数,并且定期降低限额;供需法则决定了每公吨许可的交易价格。