Which of the following describes a properly controlled experiment?
A. Fifteen turtles are fed iceberg lettuce and vitamins, while another
fifteen turtles are fed only iceberg lettuce.
B. Twenty mice are fed doughnuts every day and their weight is
measured after two months.
C. A poll is taken to determine the most popular website ever.
D. Five radish seeds are planted in an outdoor garden that gets lots
of sun and water.