Throttle Body Injection:
Carburetors were first replaced by throttle body injection (also know as single point or central fuel injection). This system can use a single or multiple electrically controlled fuel- injectors located above the throttle body, the same location as was used by carburetors. For this reason, these were almost a bolt-in replacement for the carburetor allowing throttle body injection to be simple explained as being a computer controlled carburetor. Automakers didn't have to make any drastic changes to their engine designs and so components such as the air cleaner, intake manifold, and fuel line routing could all be reused. And so the justification for single point injection was the low cost. These systems were not around very long only acting as a transition from carburetors to multi-port fuel injection.