If you have 40 milligrams of uranium-232 that has a half-life of 70 years, how much of uranium-232 will remain in 140 years?

The correct answer is Option D.

Explanation:

Mixture is defined as the combination of two or more different substances in which each substance retains its original chemical properties. These can be physically separated.

Pure substance is defined as the substance which is formed by only one type of atom or only one type of molecule. The atoms forming a pure substance loose their original chemical properties.

Sodium chloride is the compound which are formed by the chemical combination of sodium and chlorine atoms.

When a compound is formed, the elemental properties of the atoms are lost and the compound formed attains new chemical and physical properties. The compound formed will be a pure substance.

From the above information, the correct answer is option D.