Which of the following statements describes a difference between a mixture and a pure substance?
A.The composition of a mixture can vary from sample to sample, but the composition of a pure substance is always the same.
B.The density of a mixture can change with temperature, but the density of a pure substance cannot change.
C.A mixture tends to be homogeneous, while a pure substance tends to be heterogeneous.
D.A mixture has a specific melting point, while the melting point of a pure substance can vary.