Two spies have to communicate using a secret code. they need to create exactly 30 possible precoded messages, using a single number and letter. which structure should the code have?

a.
select a number from {1, 2, 3, 4} and a vowel.

b.
select a number from {1, 2, 3, 4, 5} and a vowel.

c.
select a number from {1, 2, 3, 4, 5, 6} and a vowel.

d.
select a number from {1, 2, 3, 4, 5} and a consonant.