Problem
According to a report, the standard deviation of monthly cell phone bills was $\$ 4.85$ in 2017 . A researcher suspects that the standard deviation of monthly cell phone bills is different today. (a) State the null hypothesis in words. Choose the correct answer below. A. The standard deviation of monthly cell phone bills is $\$ 4.85$. B. The standard deviation of monthly cell phone bills is different from $\$ 4.85$. C. The standard deviation of monthly cell phone bills is greater than $\$ 4.85$. D. The standard deviation of monthly cell phone bills is less than $\$ 4.85$.
Solution
Step 1 ：The null hypothesis is a general statement or default position that there is no relationship between two measured phenomena, or no association among groups. In this case, the null hypothesis would be that the standard deviation of monthly cell phone bills is still $4.85, as it was in 2017. Therefore, the correct answer is A. The standard deviation of monthly cell phone bills is $4.85.
Step 2 ：Final Answer: \(\boxed{\text{A. The standard deviation of monthly cell phone bills is \$4.85}}\)
