Identify the following data as either time series or cross-sectional.
a. The number of rooms booked each night for the month of January 2010 at a Vail Resorts hotel.
b. The amount spent on books at the start of this semester by each student in your statistics class.
c. The number of Caesar salads sold for the week of April 19, 2010, at each Noodles & Company restaurant.
d. The stock price of Coca-Cola Enterprises on May 1st for each of the last 10 years.