This strengthens by eliminating the possibility that fairy circles are formed by above-ground animals eating the grass. If the dying plants are damaged only at the roots, the cause is likely something underground.

But why would circular patches without grass form? (B) doesn’t help establish that the cause is likely underground. It also doesn’t eliminate drought as an explanation for the circles, because drought isn’t something that would have created circles in the first place.

If anything, this might weaken the argument by providing an alternate explanation for the formation of fairy circles — maybe it has something to do with the higher water content of the soil.

This gives us information on where fairy circles tend to form. But this doesn’t suggest anything about the cause of fairy circles. Are they due to termites? Or something else?

We already know from the premises that sand termites are found in every fairy circle investigated. We don’t need additional support for the presence of the termites near fairy circles. The issue is whether the termites that we know are there are the cause of fairy circles.