Because a (when it means 'and') changes to ac if it comes before a word starting with a vowel - a e i o u w y are all vowels in Welsh, remember.
(Except if it comes before y/yr ('the'), in which case a + y/yr becomes a'r.)
Cool, i get it, Like in English when the word 'a' turns to an 'an' if it's before a vowel, so it avoids the clash of two vowels after each other. Thanks!
It should accept 'a(n)' instead of just 'one', they both mean a singular thing