The correct answer is - pull factors.
The increased freedom and much more employment opportunities in the United States can be considered as pull factors. They can be considered as pull factors because they are big reasons why people from all around the world got attracted to the idea of migrating to the United States, as they saw a nice chance to prosper and have a better life. Most of the countries were still struggling with the freedom issues, and the employment opportunities were either very limited, or badly payed, so no wonder that pople wanted to move in the United States.