The number of people served by advanced wastewater treatment plants doubled between 1968 and the mid-1990s, but because the treatment plants__________, the quantity of waste emitted into the water after treatment _____________.
a. became so much more effective; decreased by about one-third
b. became so much less effective; increased by about one-third
c. remained ineffective; remained the same
d. moderately improved; increased by 10%