Biologists have ascertained that specialized cells convert Chemical energy into mechanical energy.

单项选择题As nineteenth-century American cultural aspirations expanded, women stepped into a new role as interpreters of art, both by writing works on art history and by teaching art.

单项选择题Nitrogen is the most plentiful gas in the atmosphere.

单项选择题Before herbs were available in supermarkets year-round, herb vinegar was made in the fall.