GMAT Critical Reasoning (CR) | Networks of blood vessels in bats wings serve only

Here is the problem with B:

"All creatures that fly by flapping their wings have networks of blood vessels in the skin of their wings."

This is a typical logic trick. Basically, it is testing whether situation A (flapping wings means blood vessels are present) rules out situation B (gliding means blood vessels are not present). Clearly, situation A being truedoes not rule out situation B. Knowing something about A is not sufficient to make a determination of characteristics of B. If
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