Announcements for 85 upcoming engineering conferences were randomly picked from a stack of IEEE Spectrum magazines. The mean length of the conferences was 3.94 days. The population standard deviation is 1.29 days. Assume the underlying population is normal. Part (a) In words, define the random variables X and X. X is the mean length from a sample of 85 engineering conferences, and X is the length of an engineering conference. X is the length of an engineering conference, and X is the mean length from a sample of 85 engineering conferences. X is the number of engineering conferences, and X is the mean number of engineering conferences listed in IEEE Spectrum magazine. X is the mean number of engineering conferences listed in IEEE Spectrum magazine, and X is the number of engineering conferences