The centers of most cities became business areas after the invention of
what?

streetcars

Explanation:

When the streetcars were invented, many positive economic impacts turned cities centres into business areas.

The streetcar enabled opportunities to connect people and set an environment likely for carrying out business:

As traditional downtowns had heavy roads traffic, the streetcar made possible for people to quickly come to meet in the centre of the city.

Once a streetcar system was implemented, the average foot traffic enormously increased along its route. And that meant that more people walking, more pockets to spend at the local business along that road.

The same effect has been noticed in many cities, not only in the context of US or Europe but in most of the world.

Some cities came late to begin using a streetcar system, and yet their economic impact hast generally turned predictably good.

A so-called domino effect occurred, and further economic activities were triggered along that way.