Is the Mean Age at Which U

Is the mean age at which U.S. children learn to walk less than 15 months? A study of 40 U.S. children found a mean walking age of x̄= 13.2 months. If the population of all U.S. children has mean age μ until they begin to walk and standard deviation σ, which of the following null and alternative hypotheses should we test to answer this question?

A) H₀: μ= 13.2 versus H_a: μ< 13.2
B) H₀: μ= 13.2 versus H_a: μ≠13.2
C) H₀: μ= 15 versus H_a: μ≠15
D) H₀: μ= 15 versus H_a: μ< 15