Question

1. we know these tribes exist because...

a. villages have been spotted by plane.
b. anthropologists track them.
c. they are under close surveillance.
d. all of these

Explanation:

To determine the answer, we analyze each option:

• Option a: If villages of these tribes have been spotted by plane, that's a way to know they exist.
• Option b: Anthropologists tracking them would provide evidence of their existence.
• Option c: Being under close surveillance also implies their existence is known.

Since all these reasons (a, b, c) contribute to knowing the tribes exist, the correct option is d.

Answer:

d. all of these