I think A.
(A) Sandactylus would not have had networks of blood vessels in the skin of its wings if these networks were of no use to Sandactylus.
Yep-- The assumption is that if it worked in a fashion for "the bats" then it works with the similar fashion for Sandactylus
(B) All creatures that fly by flapping their wings have networks of blood vessels in the skin of their wings.
--This is not an assumption this could be a fact or premise.
© Winged dinosaurs that flapped their wings in flight would have been able to fly more effectively than winged dinosaurs that could only glide.
-- effectiveness ???
(D) If Sandactylus flew by flapping its wings, then paleontologists would
certainly be able to find some evidence that it did so.
--I dont think they have evidence thats why they are hypothesizing
(E) Heat generated by Sandactylus in flapping its wings in flight could not have been dispersed by anything other than the blood vessels in its wings
--This would be a nice finding or premise
Whats the OA ??