Which two elements make up the greatest percentages by mass in Earth's crust?

• A. Oxygen and silicon
• B. Magnesium and potassium
• C. Iron and aluminum
• D. Calcium and sodium

Answer: (A)

Verified step by step guidance

1. Understand that Earth's crust is composed of various elements, but some are much more abundant by mass than others.
2. Recall that oxygen is the most abundant element in the Earth's crust by mass because it forms many compounds, especially oxides.
3. Recognize that silicon is the second most abundant element in the crust, primarily because it forms silicate minerals, which are very common.
4. Compare the given options by considering the typical abundance of these elements in the crust: oxygen and silicon are known to dominate, while magnesium, potassium, iron, aluminum, calcium, and sodium are present but in smaller amounts.
5. Conclude that the two elements making up the greatest percentages by mass in Earth's crust are oxygen and silicon.