Which of the following substances is NOT a solution (i.e., not a homogeneous mixture at the molecular or ionic level)?

• A. Saltwater (aqueous $\mathrm{NaCl}$)
• B. Sand in water
• C. Air (a mixture of gases such as $N_2$ and $O_2$)
• D. Brass (an alloy primarily of $\mathrm{Cu}$ and $\mathrm{Zn}$)

Answer: (B)