question:  Why did Pop go to Lunar City?
context: Once in two weeks Earth-time a rocketship came around the horizon from Lunar City with stores for the colony deep underground. The Crack, of course, was that gaping rocky fault which stretches nine hundred miles, jaggedly, over the side of the Moon that Earth never sees. When it was first found, scientists descended into it to examine the exposed rock-strata and learn the history of the Moon before its craters were made. He'd shipped out, whimpering, to the Moon to get far away from Pop, and Pop was just about a mile overhead and there was no way to get around him. Pop had come back to consciousness in a hospital with a great wound in his head and no memory of anything that had happened before that moment. There was a warning-bell in the shack, and when a rocketship from Lunar City got above the horizon and could send a tight beam, the gong clanged loudly, and Pop got into a vacuum-suit and went out the air lock. He usually reached the moondozer about the time the ship began to brake for landing, and he watched it come in. At first Pop followed absorbedly from city to city, to recover the years that had been wiped out by an axe-blow. When Sattell fled to another continent, Pop followed because he had some distinct memories of his wife—and the way he'd felt about her—and some fugitive mental images of his children. Even when Sattell—whimpering—signed up for Lunar City, Pop tracked him. Yet here on the Moon Pop kept a waiting cannister on a shelf in his tiny dome, behind the air-apparatus. He'd come to the Moon in the first place because Sattell was here. The occupation made it absurdly easy to live on the surface of the far side of the Moon, whether anybody else could do it or not. Living on the Moon was bad enough anyhow, then, but living one mile underground from Pop Young was much worse.
output:  Pop went to Lunar City because Sattell went to Lunar City.
options:  Pop went to Lunar City because the Earth is overcrowded.  Pop went to Lunar City because Sattell went to Lunar City.  Pop went to Lunar City because his family was murdered, and he couldn't stand to be on Earth any longer.  Pop went to Lunar City to mine diamonds.