What is a material that contains three elements joined in a fixed proportion?

A material that contains three elements joined in a fixed proportion will be a compound.

Explanation:

An element can be described as a substance that is made of only one kind of atom.

On the other hand, a compound can be described as a substance that is made up of more than one atom or elements.  A compound can contain fixed proportions of each element that it is made of or it can even contain different proportions of each element.

For example, carbon dioxide is a compound which will always contain 12 grams of carbon for every 32 grams of oxygen.

.) the observation by semmelweis that more mothers died of childbed fever when their doctors also worked in the morgue

i think it would be 5