单选题 .  ①What accounts for the low-lying, flat surface of Mars's north? ②On Earth's surface, higher- and lower-lying areas have different types of crust: one, thin and dense, is pulled toward Earth's center more strongly by gravity, and the planet's water naturally comes to sit over it, creating oceans. ③The processes that generate this oceanic crust drive plate tectonics.
    ①Is Mars's north similarly characterized by a sort of crust different from other areas of the planet? ②Some researchers do see signs of tectonic activity surrounding the northern basin that suggest that it was created through the formation of new crust, like ocean basins on Earth. ③However, McGill points to northern bedrock structures that predate the features said to mark the start of the tectonic process. ④McGill instead believes that through some novel mechanism the ancient surface sank to its current depth as a single unit. ⑤This would explain why features around the basin's edge, which would have formed as the surface dropped, seem to be younger than structures at its floor.
    ①The third possibility is that the northern lowlands result from impacts. ②Some researchers suggest they formed as a series of big overlapping impact craters. ③Others arguing that the odds against such a pattern of impacts are large, postulate a single event—the impact of an object bigger than any asteroid the solar system now contains.17.  The primary purpose of the passage is to ______