The Amazon valley is very important to the natural balance of the earth．Forty percent of the world’s oxygen     there．   

    A．produces       B．has produced      C．is produced      D．has been produced  

The Amazon valley is very important to the natural balance of the earth．Forty percent of the world’s oxygen　　　 there．

　 A．produces　　　　　　　　 　　　 B．has produced

　 C．has been produced　　　　　　　 D．is produced