Can't decide which article to use
Hello! Could you help me with choosing the right actile?
A speaker is on the phone with his friend. He wants to open a book on his laptop. He knows which book he's going to open, but his friend doesn't. Does the speaker say "Let me open a book" or "Let me open the book"?
At first, I wanted to start this post with this: "Could you help me with \_\_/an/the article choice?" Which option would have been correct? Or would you say that another way? "A choice of an article"? "The choosing of an article"?
When talking about levels of English, do I say "He's at \_\_/a/the B2 level"?
If I talk about wanting to take \_\_/a/the FCE/CAE/CPE/GMAT/TOEFL exam, which article do I use?
I'd greatly appreciate it if you also explained each of your choices.