I might be wrong, but I think the error is the way they spelled okay. When you are writing in an essay or any other forms of writing, I think you’re supposed to spell the word out instead of abbreviating it.
I hope this helps :)
She was wondering whether they've (A) completed their lifeguard training.
Answer:
3 and 13
Explanation:
the period number of aluminum is 3. it's a metal from group 13.