Experiments on learning in animals sometimes measure how long it takes mice to find their way through a maze. The mean time is 18 seconds for one particular maze. A researcher thinks that a loud noise will cause the mice to complete the maze faster. She measures how long each of 10 mice takes with a noise as stimulus. The appropriate hypotheses for the significance test are (a) $$H_0: \mu = 18; H_a: \mu \neq 18.$$ (b) $$H_0: \mu = 18; H_a: \mu > 18.$$ (c) $$H_0: \mu < 18; H_a: \mu = 18.$$ (d) $$H_0: \mu = 18; H_a: \mu < 18.$$ (e) $$H_0: \overline{x} = 18; H_a: \overline{x} < 18.$$