How the hell does a 'R' have an 'A' sound?..correct! on both accounts:
We were all taught that a precedes a word starting with a consonant and that an precedes a word starting with a vowel (a, e, i, o, u, and sometimes y).
Here’s the secret to making the rule work: The rule applies to the sound of the letter beginning the word, not just the letter itself. The way we say the word will determine whether or not we use a or an. If the word begins with a vowel sound, you must use an. If it begins with a consonant sound, you must use a.
Therefor the correct term is 'an RS'. This is because the letter R at the beginning has an A sounds which is a vowel and 'an' is correctly used before RS.
Got it kids!?