The correct answer is b).
Since temperature changes are continuous, we need to use something that changes continuously with the temperature.
a) A metal bar expands (changes its length) as it is heated, so we could use the changing length as a measure of temperature.
b) A liquid that changes colour suddenly at 100 °C would not be very useful.
All it would tell you is when the temperature is 100 °C.
c) According to Gay-Lussac's Pressure Temperature Law, the pressure
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Thus, we could use the pressure as a measure of the temperature.
d) Liquid mercury expands as the temperature increases.
Thus, we can use mercury in a thin tube to measure temperature.
As the mercury in the thermometer bulb expands, it moves through a thin capillary graduated in degrees.
The clinical thermometer above shows a temperature of 36.5 °C.