【单选题】听力原文: No living creature, plant or animal, can exist in complete isolation. An animal is bound to depend on other living creatures, ultimately plants, for its food supply it must also depend upon the ...
A.
They eat or destroy plants.
B.
They provide plants with continued oxygen supply.
C.
Their manure is eaten by plants.
D.
Their manure improves the quality of soil which plants live on.