Consider a set U of 23 different compounds in a Chemistry lab. There is a subset S of U of 9 compounds, each of which reacts with exactly 3 compounds of U. Consider the following statements:

I. Each compound in U S reacts with an odd number of compounds.
II. At least one compound in U S reacts with an odd number of compounds.
III. Each compound in U S reacts with an even number of compounds.

Which one of the above statements is ALWAYS TRUE?