You are asked to write a program that will display a letter that corresponds with a numeric rating system. The program should use a switch statement. The numeric rating is stored in a variable named rate and rate may equal 1, 2, 3, or 4. The corresponding letter is stored in a variable named grade and grade may be A, B, C, or D. Which is the test expression for this switch statement?

a. rate
b. grade
c. 1, 2, 3, or 4
d. A, B, C, or D

a. rate