TOEFL Reading: ETS-TOEFL阅读机经 - KK846807U5G21XL3L$

Why does the author use the phrase "the fuel on which ecosystems run?" A. To explain why grasslands are a difficult ecological pattern to study B. To illustrate what discovery scientists made at the LTER site in the Flint Hills of northeastern Kansas C. To point out that research run by the LTER program can lead to practical results D. To emphasize the fundamental importance of precipitation on ecosystems