Which statement contrasts the two types of mixtures? a. a heterogeneous mixture has a uniform composition, whereas a homogeneous mixture has visually distinguishable parts. b. a heterogeneous mixture has visually identifiable parts, whereas a homogeneous mixture has a uniform composition. c. a solution is composed of a fixed ratio of elements forming a compound, and a heterogeneous mixture has a uniform composition. d. a solution contains a solute and a solvent, whereas a heterogeneous mixture contains a mixture of solvents.