Final answer:
When two elements combine to form different compounds, both elements combine in a simple ratio, as explained by the Law of Multiple Proportions.
Step-by-step explanation:
When two elements combine to form different compounds, the correct statement is C, which is that both have a simple ratio. This is in accordance with the Law of Multiple Proportions, which states that if two elements form more than one compound between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will always be ratios of small whole numbers. This law was key in the development of the atomic theory and is fundamental in understanding how elements combine to form compounds.
For example, consider carbon and oxygen, which can form carbon dioxide (CO2) and carbon monoxide (CO). If we fixed the mass of carbon, the masses of oxygen that combine with it in CO and CO2 will be in a simple whole number ratio, reflecting the different proportions by mass in which these two elements can combine to form different chemical compounds.