proverbs - which one?

Hi folks. I´m studying proverbs by myself and there is an exercise which is troubling me. It goes like this:
A- Say Kate. Are you enjoying Mrs. Kramer´s classes?
B- Not really. You know, I don´t feel very comfortable most of the time.
A- I already know you do but __________________________.
B- Yeah, you´re right. By the way, we´re traveling for a competition in two months.

Question 1- Can I say " I already know you do or should I say I already know you don´t?
Q-2 Which proverb can I use after but?
a- Don´t burn your bridges behind you
b- one person´s meat is another one´s poison
c- I can use both proverbs.

Thanks for your help.