Why Does the Parasitic Flatworm (Leucochloridium Paradoxum) of the Amber

Why does the parasitic flatworm (Leucochloridium paradoxum) of the amber snail (Succinea putris) alter the behavior of the snail so that it moves to the top of a plant stem?

A) The flatworm needs the snail to move off the ground to avoid predation.
B) The flatworm needs the snail to be eaten by a bird to complete its life cycle.
C) The conditions at the top of plant stems are better for flatworms.
D) There is more food for the snail at the top of plant stems than on the ground.