Надо пояснить почему поставлены именно эти слова

1) Choose a or an: an opinion, an old dress, a class, a hand.

2) Choose the correct article before countable nouns.

1. I don’t have brothers or sisters.

2. I have a cousin. Do you have a cousin?

3. Write him a letter when you come back home.

4) Choose the correct article before uncountable nouns.

1. Could you pass me the salt, please?

2. The bread is stale.

3. Paper is made of wood.

5) Fill in articles before countable and uncountable nouns. Put ‘–’ for the zero article.

1. I am going out for a walk. I need some fresh air.

2. We need petrol. I hope we’ll come to a petrol station soon.

3. The soup isn’t salty enough. Could you pass me the salt, please?