30. The word "they" in line 22 refers to
(A) pregnant females and young
(B) quarry operations
(C) the value of the fossils
(D) these factors
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統計: A(9), B(2), C(0), D(2), E(0) #802768
統計: A(9), B(2), C(0), D(2), E(0) #802768