By studying fossils, scientists have learned that
A. plants have changed over time, but animals have not.

B. animals have changed over time, but plants have not.
C. neither animals nor plants have changed over time.

D. both animals and plants have changed over time.
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D

Explanation:

both plants and animals have evolved over time

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.