Consider the following code snippet. What should be placed in the blank to cause a message to be displayed when the user enters the same letter twice in a row?

letter = input("Enter the next letter in the alphabet: ")

while letter != "":

previous = letter

letter = input("Enter the next letter")

if ________________________ :

print("Duplicate input")

Select one:

a. alphabet[0] == previous

b. alphabet[0] == letter

c. letter == previous

d. letter == letter