During which phase of product development do companies introduce their product model to actual customers

screening/evaluation
business analysis
market testing
commercialization

The answer would be market testing, hope I helped!!

People who utilizes the progression principle will ensure their exercise program increases in frequency, intensity, time, and type. the changes need to happen gradually in order to ensure they make gains in their fitness program. a person who doesn't utilize the progression principle can still gain health benefits from exercising, but will not continue to improve in cardiovascular fitness.