What is the approximate thickness of the Earth's crust?

The Earth's crust varies in thickness depending on whether it is continental or oceanic. On average, the thickness of the crust is approximately between 5 to 70 kilometers, with oceanic crust being thinner (about 5 to 10 kilometers) and continental crust being thicker (averaging around 30 to 50 kilometers).

When considering the choices, the approximate thickness range of 8-40 km provides a reasonable representation of these averages for both oceanic and continental crusts. This range encompasses the more common thicknesses found in many regions without exceeding typical measurements, making it an appropriate selection based on the general understanding of Earth's crust dimensions.

This understanding clarifies the reason other ranges may not align as accurately. For instance, the option suggesting 20-50 km is somewhat limiting and doesn't account for the thinner oceanic crust. Similarly, the selections indicating greater thicknesses, such as 40-100 km, extend beyond the average crustal measurements typically observed.