Scientists have concluded that the uppermost part of the mantle is partially-molten. Which observation helped them reach this conclusion? Both P and S waves slow down when they reach this layer.

Both P and S waves speed up when they reach this layer.

P waves do not travel through this layer.

S waves do not travel through this layer.

Both P and S waves slow down when they reach this layer

Explanation:

The asthenosphere is the uppermost part of the mantel and is also called the magma chamber. This layer s partly molten in the state is tectonically weak and is a ductile region. It almost solid and seismic waves tend to be relatively slow through the asthenosphere. The upper-most part of the asthenosphere is believed to be of great rigidity and the brittle lithosphere.