I'm confused by the sentences "-What's this? -It's V."in our textbook.As I know, letters are countable, right? But why there isn't a "a" in front of the"V"?Shouldn't it be "It's a V."?

In the meantime, there are exercises in our exercise books like: There's ___ F in the alphabet.

A.a B. an C./

And the answer to the question is "an".

And then I saw such an exercise: This is ___V. A. a B. an C. /
I think both A and C are acceptable.

Those really confuse me.Hope to get your answers soon. Thanks a lot!

p.s. I know that F is headed with the vowel [e] so the article should be 'an' while if it's a consonant it should be an 'a'.I'm just confused if an article should be used or not.