Why does the author mention "present-day volcanoes on Earth"? A. To provide an example of an important difference between present-day Venus and present-day Earth B. To help explain why Earth's atmosphere still contains traces of sulfur dioxide but Venus' does not C. To indicate one source of information about the likely composition of the original atmospheres of Venus and Earth D. To account for the fact that Earth's water supply no longer comes primarily from impacting comets