Why do we think Mercury has so many tremendous cliffs? A. They were probably carved in Mercury's early history by running water. B. They are almost certainly volcanic in origin, carved by flowing lava. C. They represent one of the greatest mysteries in the solar system, as no one has suggested a reasonable hypothesis for their formation. D. They probably formed when a series of large impacts hit Mercury one after the other. E. They were probably formed by tectonic stresses when the entire planet shrank as its core cooled.