As more and more cars are produced and used, so the _____ from their exhaust-pipes contains an even larger volume of poisonous gas.

　　A. exposure  B. distribution  C. expansion  D. emission

As more and more cars are produced and used, so the _____ from their exhaust-pipes contains an even larger volume of poisonous gas.

A. exposure  B. distribution  C. expansion  D. emission

As more and more cars are produced and used, so the _____ from their exhaust-pipes contains an even larger volume of poisonous gas.

A. exposure  B. distribution  C. expansion  D. emission

As more and more cars are produced and used, so the _____ from their exhaust-pipes contains an even larger volume of poisonous gas.

　　A. exposure  B. distribution  C. expansion  D. emission

As more and more cars are produced and used, so the _____ from their exhaust-pipes contains an even larger volume of poisonous gas.