Answer:
The efficiency of the assembly line.
Step-by-step explanation:
Since the assembly line was able to take care of things with ease, less labor was required to make these things. This meant that the workers did not need to be paid as much. And since not much effort was required to make the things, the cost could lower as time went on. On a side note, this is also why Ford's sales tripled; their things became much more affordable compared to how much they costed previously.