A​ start-up company built a database of customers and sales information. For each​ customer, it recorded the​ customer's name, zip​ code, region of the country​ (East, South,​ Midwest, West), date of last​ purchase, amount of purchase​ (in dollars), and item purchased. (a ) Identify whether the data are cross sectional or a time series. (b )Give a name to each variable and indicate if the variable is​ categorical, ordinal, or numerical​ (if a variable is​ numerical, include its units if​ possible). (c )List any concerns that you might have for the accuracy of the data.