Is there someone that is in the English major?

I need help to understand a homework problem. I already looked online and in my notes.

Question: Pick the correct sentence.

Sentence 1: The doctor studied my x-rays and then she let out an ominous sigh.

Sentence 2: The doctor studies my x-rays then she let out an ominous sigh.

I don't know which one is correct. The only difference is "and then" and "then"

Thank you,Kelly! You're absolutely right that I should be trying to figure it out. I have tried! I'm so confused on that slight difference.

@Amanda C-What's the difference between "then" and "and then"? I have looked online to figure this out but I can't find it anywhere yet I'm still trying to understand.

Thank you everyone! Thank you, Amanda C! I understand now! Thank you for making me understand! The correct sentence is the one with "and" in it. I will write what you said in my notes for future reference! Thank you so much :)