Lake Michigan is the only one of the Great Lakes that is entirely within the United States; the others form a water boundary between the United States and Canada.
I'm unsure, but it could be caused by space particles that land on it. Things such as space dust or meteorites.
The answer for your question is letter A
A. Massai
The Massai people are a seminomadic group, who are herder-warriors of the east African plains, they inhabit South Kenya/Northen Tanzania. The San Bushmen inhabit Southern Africa, and are not seminomadic; the Ga people are an ethnic group of Ghana and Togo, meaning this cannot be the answer as it is in West Africa; and the Zulu people again are South African and not nomadic.