C vs D

Because we want to weaken Galina's argument, the correct answer will need to cast doubt on Galina's claim that the squirrels are after "something other than sugar." One way to do that is to show that the squirrels are after the sugar, even if the concentration of the sugar in the maple sap is low. (D) shows how the squirrels are after the sugar, even if the concentration of the sugar in the maple sap is low. If they wait until the water evaporates to consume the sap, then they're not consuming the sap when the concentration of sugar is low. So they don't have to drink an "enormous amount of sap" to get a significant quantity of sugar. (C), on the other hand, doesn't show that the squirrels are after the sugar when they tap the maple trees. Yes, the squirrels may tap the trees with less sugar less often, but they're still going through the effort of tapping those trees. And if those trees bear even less sugar, then that could suggest that the squirrels are after something other than sugar. Maybe the squirrels want a third thing (other than sugar and water) in both of sugar maples and these other trees. And there's more of this third mystery thing in sugar maples, so they tap sugar maples more often. If these were true, it would strengthen — not weaken —  Galina's argument.