Here are the answers to your questions:
Question 1
A) The United States involved itself in Vietnam in the 1960s because:
It feared that if Vietnam became communist then surrounding countries would quickly follow.
B) How did the domino theory influence the United States' role in the Vietnam War?
It convinced the United States to become involved in the war.
Question 2
A) How does overpopulation in Indonesia affect the availability of clean water?
It increases pollution and reduces access to clean water.
Question 3
A) Why did the first inhabitants of mainland Southeast Asia settle in plains and river valleys?
Those areas were good for farming.
Question 4
A) What is a primary characteristic that differentiates religious groups from ethnic groups?
Religious groups are defined by shared beliefs and practices related to spirituality.
Question 5
A) What was the underlying belief that justified U.S. involvement in both the Korean and Vietnam Wars?
The thought that communism was a threat to democracy and freedom.
Question 6
A) What was a negative effect of the United States' involvement in the Vietnam War?
The death toll for American soldiers was high, and the American public were divided in their support of the war.
Feel free to ask if you have more questions!