A glass tumbler containing hot water is kept in the freezer compartment of a refrigerator (temperature $(<$ $0^{\circ} \mathrm{C}$ ). If you could measure the temperature of the content of the tumbler, which of the following graphs would correctly represent the change in its temperature as a function of time.
The graph (a) gives the correct representation. The temperature of water in the tumbler kept in the freezer compartment gets lowered with time till water changes into ice. At this stage, the temperature becomes $0^{\circ} \mathrm{C}$ and remains constant till the entire water in the tumbler has frozen. If the tumbler is still kept in the freezer, the temperature will fall below $0^{\circ} \mathrm{C}$ (negative sign) with passage of time.