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.

单项选择题Scientists, who are now aware of how nautiluses regulate their buoyancy, have been able to dispel erroneous ideas about these creatures.

单项选择题George Washington Carver was esteemed for his contributions in the fields of botany and chemistry.

单项选择题Pendulums are used to regulate motion in some scientific equipments.