Why is the runoff from fertilized agricultural fields, even if free of pesticides, often harmful to the ecosystems of temperate lakes?

A - The fertilizer promotes the growth of fish to the point where they cannot find enough food to eat.
B - The runoff causes a surface algal bloom, which reduces the penetration of light into the eater which reduces photosynthesis which reduces the amount of oxygen in the water.
C - Fertilizer runoff promotes antibiotic resistance.
D - The runoff raises the levels of inorganic nutrients in the surface waters to levels that are toxic for algae and other lake organisms.

What has happened to fish populations as a result, primarily, of commercial fishing activity?
A - Some commercial fish species are exhibiting faster growth rates.
B - There has been a decline in population sizes of commercial fish stocks.
C - There has been a decrease in the levels of toxins found in the flesh of fish that we eat.
D - There has been directional selection favoring larger fish.