This fossil trilobite and this living blue crab both have a limb structure called a biramous limb. What best explains why both species have biramous limbs? A. It is impossible to say. Fossils are very old, so we cannot make observations of the trilobite’s ancestors, and we cannot explain its body structures.

B. It is impossible to say. Fossils are very old, so we cannot make observations of the trilobite’s ancestors, and we cannot explain its body structures.

C. All species have their own specific body structures, so it is a coincidence that this trilobite and blue crab each happen to have the biramous limb structure.

D. The trilobite and blue crab are different species, so they must not share an ancestor population. They inherited their biramous limb structure from separate ancestor populations.

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