Which of the following is a pure substance? pizza
pizza

salt (NaCl)
salt (NaCl)

air
air

sweet tea
sweet tea

salt (NaCl)

Explanation:

Pure substance is defined as the substance that consist of a single type of material and does not contain contaminants. A pure substance has uniform composition throughout the process.

sodium chloride (NaCl) also known as table salt is a pure substance because it has a uniform and definite composition that is  1:1 ratio of sodium and chloride ions.

Hence, the correct answer is "salt (NaCl)".

altitude, density, pressure

air gets less dense the higher the altitude and also the air pressure is less as altitude rises.

a) the particles of the chair are strongly held together.