I would say that housing availability, inadequate services, pollution and migration from rural areas all present challenges to the regional urban areas but not reduced population. If there is migration to the cities then these above three problems must be dealt with to accommodate the new citizens.
Manned—
Apollo 11
International space station
Skylab
Unmanned—
Galileo
Mars exploration rover
Hubble space telescope
An homeless person makes between $15-$30 per hour, but it is usually closer to $15 per hour on average
Your answer would be D because it is the only answer choice that would attract people.