Suppose a business is trying to determine in which city (or cities) it should locate a regional office. They want to make sure that a regional office is no more than a 6-hour drive from any of the four cities (cities A, B, C, and D) in which it has a retail store. The travel time in hours between each city is shown below. Write a pure-0/1 integer program to determine the cities in which a regional office should be located, such that the number of regional offices is minimized