Điền a/an vào chỗ trống cho đúng: pencil elephant _box _ girl umbrella man _ apple _eraser _ old woman 10 _ young girl 11 _ doctor 12 _ engineer 13 _ baby 14 _ ant 15 egg 16 duck 17 bee 18 insect 19 _ fish 20 boy 21 _ long ruler 22 23 24 25 26 27 28 29 30 ink tree _ orange rabbit mouse _ yellow duck orange bird black cat jug