Basics about Opportunity Cost

May be I re-phrase my question:

In a bartering market, I have a grapefruit and have two choices only: either A or B
choice A: grapefruit exchange for 5 apples
choice B: grapefruit exchange for 3 bananas

Op. Cost of taking choice A will be 3 bananas. That's simple and direct.

Now, what will be the new Op. Cost if choice A is changed to:
(i) a grapefruit exchange for 6 apples
(ii) a grapefruit exchange for 4 apples
(iii) ($1 + one grapefruit) exchange for 5 apples
(iv) a grapefruit exchange for (5 apples + $1)

Thanks in advance again.