2. Which of the following best describes the theme of the passage?
a. The agricultural revolution changed human food preferences.
b. Some plants today are similar to their ancestors.
c. Farming replaced hunting and gathering,
d. Agricultural communities developed from hunter-gatherer communi-
ties, and the practice of agriculture introduced new environmental
problems.

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.