Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. A. Because physical and chemical properties of substances are unchanging, they are useful records of climate fluctuations over time. B. For hundreds or thousands of years, people have been observing changes in the chemical and physical properties of substances in order to infer climate change. C. Because it takes long periods of time for the climate to change, systematic changes in the properties of substances are difficult to observe. D. Changes in systematically deposited substances that are affected by climate can indicate climate variations over time.