Is the term for all the internal and external changes involved in becoming physically an adult

The correct answer is puberty.

Puberty refers to a period in life when individuals undergo significant changes and development both internally and externally. Essentially, puberty marks the time when individuals reach sexual maturity and are able to reproduce. In males and females, puberty changes not only their appearance, but also their behavior, preferences and urges. 

The changes to agriculture in the united states since 1900 is that increased mechanization has increased food production, thereby increasing the need for labor. there has been an increased need for higher speeds of food production due the fact that the population in the united states continues to rise.