Give meaningful identifiers for the following variables and write a proper C++ declaration and initialization to a reasonable value. (1, 6, 7) A variable to store the letter grade, (A, B, C, D, or F). a. A variable to store the sales tax price of an item. b. A variable to store the number of juice bottles sold. C. A variable to store the billing amount of a doctor's visit. d. A variable to store the grade point average (GPA). e