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A company wants to know how much sleep middle schoolers get at night. Which sample is the least biased?
A) Ask all the 8th graders in Tennessee.
B) Ask all the 6th graders in Texas.
C) Ask all the 6th, 7th, and 8th graders in Iowa.
D) Ask a random sample of 6th, 7th, and 8th graders in every state.
D- This sample has representatives from every state and grade level.