Here's why:

* The Law of Multiple Proportions states that when two elements combine to form more than one compound, the ratios of the masses of one element that combine with a fixed mass of the other element are simple whole numbers.

In your example:

* Carbon monoxide (CO): The ratio of oxygen to carbon is 1:1 (by mass).

* Carbon dioxide (CO2): The ratio of oxygen to carbon is 2:1 (by mass).

The ratio of oxygen in carbon monoxide to carbon dioxide is 1:2, which is a simple whole number ratio. This demonstrates the Law of Multiple Proportions.