Mohorovicic observed that earthquake waves were moving faster when recorded farther away than 200km from where the earthquake started known as the P waves. This characteristics is due to?

a change in speed beacuse of the decreased density of the mantle .
an increased wavelength of the seismic energy wave.
a change in speed because of the increased density of the mantle
an error when he read the seismograph read out

a change in speed because of the increased density of the mantle