Which of the following contains only of solutions?a.salt-water, air, steelb.sand-water, steel, milkc.mercury, air, goldd.nickel, water, apple

The correct answer is a. salt-water, air, steel. Here's why:

a. Salt-water is a solution of salt in water. Air is a solution of various gases like nitrogen, oxygen, argon, carbon dioxide, etc. Steel is a solution of iron and carbon, and sometimes other elements like nickel and chromium.

b. Sand-water is a suspension, not a solution, because the sand particles are not dissolved in the water. Steel is a solution, as explained above. Milk is a colloidal suspension of fat and protein in water, not a solution.

c. Mercury is a pure element, not a solution. Air is a solution, as explained above. Gold is also a pure element, not a solution.

d. Nickel is a pure element, not a solution. Water is a solvent,