Why does the author use the phrase "Quartz is quartz"? A. To describe how the differences between Pleistocene and Cambrian quartz reveal information about dating rocks B. To point out that the chemical composition of quartz makes it more difficult to date than other rocks C. To provide an example of how regional differences in rock sequences can make a particular rock difficult to date D. To explain that rocks are difficult to use for dating because their chemical compositions always remain the same over time