A chemist analyzes an unknown liquid. The chemist takes samples from the top, middle, and bottom of the samples. Which observation would allow the chemist to reach the conclusion that the liquid could be a pure substance?

A. All three samples have different boiling points.
B. The top and middle samples have the same boiling point, which differs from the boiling point of the bottom sample.
C. All three samples have the same boiling point.
D. The middle and bottom samples have the same boiling point, which differs from the boiling point of the top sample.