The main source of downward convection flow in the mantle is called
ridge-pull
slab-pull
slab-push
ridge-push

The right option is; slab-pull

The main source of downward convection flow in the mantle is called slab pull.

Slab-pull is the force applied by a cold, dense oceanic plate descending into the mantle as a result of its own weight. This process is known as subduction. The oceanic plate has a density higher than that of the hotter mantle below it and the difference in their density causes the plate to submerge into the mantle.

affects the water bodies: excessive use of fertilizers causes eutrophication. the fertilizers which are used on plants affect the quality of land on which it is used, further causing erosion of soil. the fertilizers used on plants contain harmful chemical fumes which affect the environment.