An ornamental thermometer, commonly known as a Galileo thermometer, contains a number of spheres of hollow colored glass, representing different temperatures, immersed in a column of ethanol. A particular sphere rises from the bottom to the top of the column of liquid when the temperature falls below the value it represents. Which statement best explains why the sphere rises when the temperature falls?
a. The density of the liquid has increased
b. The mass of the sphere has decreased
c. The pressure exerted by the liquid has decreased
d. The temperature of the sphere is different from that of the liquid
e. The volume of the gas inside the sphere has decreased