According to a social media​ blog, time spent on a certain social networking website has a mean of 18 minutes per visit. Assume that time spent on the social networking site per visit is normally distributed and that the standard deviation is 3 minutes. Complete parts​ (a) through​ (d) below.

a. If you select a random sample of 25 ​sessions, what is the probability that the sample mean is between 17.5 and 18.5?  

b. If you select a random sample of 25 sessions, what is the probability that the sample mean is between 17 and 18 ​minutes?

c. If you select a random sample of 100 ​sessions, what is the probability that the sample mean is between 17.5 and 18.5 ​minutes?