With respect to the x-wind model, what is the 'x-point'?
a) Where the mantle and the core meet within a differentiated planetesimal.
b) Where the photosphere of the star meets with the accreting material.
c) The outer edge of the protoplanetary disk.
d) Where the accreting material meets the star's magnetosphere and the gas is launched outward to become the x-wind.
