Question

where is a river typically the most polluted? at the halfway point of the river there is no specific place as far upstream as possible farthest downstream

Explanation:

Pollution sources (like industrial waste, sewage, runoff) enter rivers upstream. As water flows downstream, pollutants accumulate (from multiple sources along the way) and are carried further. So the farthest downstream part has the most accumulated pollution. The other options are incorrect: halfway point doesn't accumulate all upstream pollution, "no specific place" is wrong as downstream accumulates more, and upstream is where pollution starts (not most polluted yet).

Answer:

farthest downstream