This exercise below is from my English book. I noticed that the author took the example from Macmillan Dictionary here (1.b)
They_____good friends, but they’ve fallen out recently.
a. used to be b. would be c. were d. are
The given answer is (a), but I'm not sure if (c) can be a valid answer in this case. Is there any difference in meaning if I replace 'used to be' with 'were' in this sentence? Thanks