According to the author, why is it difficult to explain the reasons for Maya collapse?

• A. Copan inhabitants destroyed all records of that period
• B. The constant deforestation and hillside erosion have wiped out all traces of the Maya kingdom.
• C. Archaeological sites of Mayas do not provide any consistent evidence.
• D. It has not been possible to ascertain which of the factors best explains as to why the Maya civilization collapsed.