Shots of Grammar: A or ONE?

Complete the sentences. A or ONE?

1__ day last year - it was 2__  very hot afternoon in June - I was hurrying to get home. I was about 3__ hour late - well,to be precise, exactly 4__ hour and ten minutes: I had taken the train that arrived at the station at 6:15. Anyway, there was 5__ woman standing under the trees, and there were several children with her. I saw 6__ child clearly - she was 7__ lovely dark-haired girl - but I only heard the others. Suddenly 8__ strange thing happened. The girl took some stones and leaves out of her pocket and threw 9__ stone after another into the air.

                                                                                                                                                                               

answers: 1.one 2. a 3.an 4.one/an 5.a 6.one 7.a 8.a 9.one