All gases and most liquids and solids expand when heated, but they do not expand equally.

单项选择题The Industrial Revolution marked the beginning of an epoch of exodus from rural areas to cities.

单项选择题City dwellers are exhilarated by country air.

单项选择题Enamel, the hardest substance in a human being’s body, covers the entire crown of the tooth.

单项选择题The five classic foot positions in ballet are the basis for the ethereal grace of the ballet dancer’s art.

单项选择题Molly Brown was labeled “unsinkable” after she helped to evacuate passengers from the ill-fated ship the Titanic.