3. all three phases can exist in equilibrium with one another at the of a substance. triple point melting point critical point boiling point

triple point.

At the melting point can exist two phases in equilbrium (liquid and solid).

At the boiling point can exist two phases in equilibrium (liquid and gas).

The critical point is the limit at wich gas and liquid can coexist.

And, indeed, at the triple point can solid, liquid and gas can coexist.

Hi. where is the graph?

The correct answer is d