Astronomers estimate that the runoff channels seen on Mars must be at least 3.9 billion years old. How do they estimate ages like this?1) by examining radioactive rocks from these channels returned to Earth by robot probes2) by counting impact craters in the regions containing these channels3) by looking for the erosion of these channels by the regular periods of rainfall that occur on Mars even today4) by measuring the height of the region in which the channels occur; on Mars, older regions are always lower in elevation5) guessing

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