B"Most of the decline in industrial discharges of
lead occurred before 1976, but the largest
decline in the level of river contamination
occurred between 1980 and 1985."
reason -It shows regulation on discharge has a little effect on contamination level of river.That's why no sign of significant decline saw in early years of 1976.
Unleaded fuel comes in picture gadually so effected lately...
Comeup with flaws please.