Explanation

(c) In the $p - T$ diagram of water, if the three curves $AB,\ CD$ and $EF$ are extended, they came to meet at a point P, called the triple point.
Therefore, triple point of a substance is a point in the phase diagram representing a particular set of pressure and temperature at which the solid, liquid and vopour phases of the substance can coexist.
For water, the values of pressure and temperature corresponding to triple point P are 0.46 cm of mercury and 273.16 K (or 0.01$℃$) respectively.
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