Option 1 states that heavy water used in a nuclear reactor is water with a molecular weight of 18 amu. However, this is incorrect. Heavy water is actually characterized by water molecules with a higher molecular weight, specifically deuterium, which has a mass of 2 amu.

Option 2 correctly states that heavy water used in a nuclear reactor is water with a molecular weight of 20 amu. This is because heavy water contains deuterium, an isotope of hydrogen which has a molecular weight of 2 amu, compared to regular hydrogen`s molecular weight of 1 amu. Since water consists of two hydrogen atoms and one oxygen atom, the total molecular weight of heavy water is 20 amu (2 x 2 amu + 16 amu).

Option 3 suggests that heavy water is water at 4°C but with a molecular weight of 19 amu. However, this is incorrect as well, as heavy water is distinguished by its higher molecular weight due to the presence of deuterium.

Option 4 states that heavy water is found below the ice in a frozen sea. While it is true that heavy water can be found in natural sources such as oceans, lakes, and even ice, the defining characteristic of heavy