Given scientific fact as input, generate the question from this fact such that it can be answered from the input.

Input: Consider Input: The sum of the masses of the atoms in the formula is referred to as molecular mass.

Output: The sum of the masses of the atoms in the formula is referred to as what?

Input: Consider Input: Urea is made in the liver and excreted in urine.

Output: What is made in the liver and excreted in urine?

Input: Consider Input: Ash that enters the air naturally as a result of a volcano eruption is classified as a primary pollutant.
Output: Ash that enters the air naturally as a result of a volcano eruption is classified as what kind of pollutant?