question:  Why did the citizens of Superior fear the edge of the town?
context: Radio messages stated simply that Superior had seceded from Earth. But Don Cort, stranded on that rising town, was beginning to suspect that nothing was simple about Superior except its citizens. The state police converged on the former site of Superior from several directions. Communicating by radiophone across the vast pit, they confirmed that the town undoubtedly was missing. The train's schedule called for it to pass through but not stop at Superior at 11:58. Someone pointed out that October 31 was Halloween and that midnight was the witching hour. A National Guard officer volunteered to take a jeep down into the pit, having found a spot that seemed navigable. A United Airlines pilot found Superior early on the morning of November 1. The object loomed out of a cloudbank at twelve thousand feet and Studley changed course to avoid it. A few minutes later he had relayed a message from Superior, formerly of Ohio, addressed to whom it might concern: It said that Superior had seceded from Earth. One other radio message came from Superior, now airborne, on that first day. Don made his way up to the locomotive and found a bigger knot of people gathered in front of the cowcatcher. Some sort of barricade had been put up across the tracks and it was covered with every imaginable kind of warning device. Don Cort and the fireman walked cautiously toward the edge while the professor ambled ahead with the familiarity of one who had been there before. Nevertheless, Don could see that it apparently was a neat, sharp edge, not one of your old ragged, random edges such as might have been caused by an explosion. He didn't have to stand on tiptoe any more than he had to sit on the edge of his seat during the exciting part of a movie, but the situation seemed to call for it.
output:  They feared that they would plummet to the ground because of gravity.
options:  They feared that they would plummet to the ground because of gravity.  They feared they would fall with the flow of the stream.  They feared they would vanish if they left the edge.  They feared they would be sucked into a vortex.