Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

Professor, I presume you know to use "a" before consonants and "an" before vowels, but what you apparently don't know is that the usage is determined by the pronunciation- not the spelling. 

The word "one" has a consonant sound, the same as the word "won." Therefore, the correct usage is "a One Hundred!"

And the truth is, that one really jumps out at you. To say "an One Hundred" is so bad- the very sound of it is like nails scraping the chalk board- that you should be required to take Bonehead English, like some freshmen are at the start of college. 

So, you're score on that is a zero. Not an zero, but a zero.