Answer: swam
Explanation:
Assuming it's past tense, swam would be grammatically correct.
The first one,
Readers can use them to identify main ideas at a glance.
This isn't neccessarily true. Your introduction can still make sense without the hook, and the information inside the introduction part should really not directly depend on the hook to explain them, they should either be self explanatory, or you should explain them there.
Hopefully this helps!
