В ответах на это задание разговаривают, что мы должны поставить артикль

В ответах на это задание разговаривают, что мы обязаны поставить артикль "a" под номером 10. Подскажите пожалуйста, почему? Речь же идет о определенном петухе, который гуляет.

1) Frenchman was once travelling in (2) England. He did not know (3) English quite well. He could speak only (4) little. One day he was eating in (5) small pub and he wanted to order (6) eggs. But he didnt know (7) English word for (8) eggs. Suddenly through (9) window he saw that (10) rooster (петушок) was walking in (11) yard. He asked (12) waiter what was the English for (13) roster. (14) waiter told him. (15) Frenchman then asked what was the English for (16) roosters wife. (17) waiter told him that it was (18) hen. Next (19) Frenchman asked what was the English for hens children. (20) waiter told him that they were (21) chickens. (22) Frenchman asked what (23) chickens were before they were born. (24) waiter told him they were (25) eggs. Youve explained everything very well, (26) Frenchman said. Please bring me two (27). eggs and (28) cup of (29) coffee.