Which two elements are the most abundant by mass in Earth's crust?

• A. Calcium and sodium
• B. Oxygen and silicon
• C. Hydrogen and carbon
• D. Iron and aluminum

Answer: (B)

Verified step by step guidance

1. Understand that the question asks for the two most abundant elements by mass in Earth's crust, which is the outermost solid layer of the Earth.
2. Recall that Earth's crust is primarily composed of oxygen and silicon, which together form many common minerals such as silicates.
3. Recognize that oxygen is the most abundant element by mass in the crust because it forms oxides with many other elements, and silicon is the second most abundant due to its role in silicate minerals.
4. Note that elements like calcium, sodium, iron, and aluminum are also present but in smaller amounts compared to oxygen and silicon.
5. Conclude that the correct answer is oxygen and silicon, as they make up the majority of the Earth's crust by mass.