a, an og the

Hej, 

Jeg sidder lige nu og laver en aflevering i engelsk. Jeg har en sætning som lyder:

"Irene Vaughan, who was Frist World War ambulance driver and later set up the first overseas operational unit in the Womens Royal Naval Service, sat her finals at Oxford in 1911, accompanied by a chaperone, but was not awarded her degree until seventy-four years later, when she was 95."

I denne sætning vil jeg rette "who was First World War ambulance driver" til "who was a First World War ambulance driver". 

Er der nogen som kan svare på hvilket ord "a" refererer til?