—Why does the river smell terrible?
—Because large quantities of water__________．

—Why does the river smell terrible?

       —Because large quantities of water__________．

       A.have polluted          B.is being polluted   C.has been polluted    D.have been polluted

--Why does the river smell terrible?

--Because large quantities of water__________.

 ---Why does the river smell terrible?

   ---Because large quantities of water          .

   A．have polluted      B．is being polluted  C．has been polluted   D．have been polluted

--Why does the river smell terrible?
--Because large quantities of water__________.