Layer A is the mantle layer, while Layer B is the crust. The crust is the top layer, and it is also the thinnest. The temperature and pressure on it are by far the smallest compared to any other layer, and this is a layer that is brittle solid. The mantle layer, on the other hand, is much hotter layer, as well as having much higher pressure. The temperatures are so high that the rocks are actually melted in the mantle, thus producing the magma.
