You are studying the use of "picture cell phones" and know that the average age of users (mu) = 20 years old, the standard deviation (sigma) = 5 years old. You sell a picture cell phone to someone who is 15 years old. Use the transformation formula to standardize this data and find the value of Z for this individual who is 15 years old. Z equals a. +1.5 b. +1.0 c. -1.5 d. -1.0