TOEFL Reading: ETS-TOEFL阅读机经 - 53W3M71374HH1DOSL$

In paragraph 1, the author introduces the concept of a budget in order to A. indicate how different cycles in the Earth system relate to each other B. illustrate how Earth's energy cycle must maintain an overall balance C. show that Earth gains energy from both external and internal sources D. explain how energy is transferred from one storage place to another