Answer:
(A) $500,000
Explanation:
Given formula for finding the cost ( in dollars ) to remove p percent of a certain pollutant,

If P = 90,

If P = 80,

∵ 900000 - 400000 = 500000,
Hence, it will cost $500,000 more to remove 90% of the pollutant from the pond than it would cost to remove 80% of the pollutant,
Option '(A)' is correct.