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 loud noise as stimulus. The appropriate hypotheses for the significance test are

a. H₀: μ = 18; H_a: μ ≠ 18.
b. H₀: μ = 18; H_a: μ > 18.
c. H₀: μ < 18;H_a: μ = 18.
d. H₀: μ = 18; H_a: μ < 18.
e. H₀: x¯ = 18; H_a: x¯ < 18.

Select the best answer.