There is little variety among organisms found in pools, ponds, and lakes.

That's true. There is little variety among organisms found in pools, pounds and lakes. Despite that fact that many ponds are seasonal, lasting just a couple of month (for exemple sessile pools) while lakes can exist for more than hundred years. Lakes and ponds have limited species diversity because they are often limited from each other and from other water sources like rivers, seas and oceans. 

The answer is d. when it begins to fall back to the child's hand

Heatamount of salt . . .