Which of the following describes a properly controlled experiment?
a. five radish seeds are planted in an outdoor garden that gets lots of sun and water.
b. fifteen turtles are fed iceberg lettuce and vitamins, while another fifteen turtles are fed only iceberg lettuce.
c. a poll is taken to determine the most popular web site ever.
d. twenty mice are fed doughnuts every day and their weight is measured after two months.