21. Why does the author mention “sterile mules” in the fourth paragraph?
(A) To delineate one of the only instances of an occurrence.
(B) To contradict the opinion presented in the passage.
(C) To provide evidence that supports scientists’ beliefs.
(D) To reiterate that the lizard is an unusual creature.

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統計： A(3), B(6), C(9), D(1), E(0) #1755949