The line connecting the triple point and the critical point on a phase diagram represents _____.

a) the temperatures and pressures above which only a supercritical fluid can exist
b) the temperatures and pressures at which the solid and gas states are equally stable and at equilibrium
c) the temperatures and pressures at which the liquid and solid states are equally stable and at equilibrium
d) the temperatures and pressures at which the liquid and gas states are equally stable and at equilibrium