punctuation geekery

someone linked to this apostrophe test on twitter last night. I took issue with one of the questions

You have to select *one* option as the correct answer. Go for it.

Question 10:
a) The Roman’s bridges and roads were vital for moving the troops’ supplies.
b) The Romans’ bridges and roads were vital for moving the troops’ supplies.
c) The Romans’ bridges and roads were vital for moving the troop’s supplies.
d) The Romans bridge’s and road’s were vital for moving the troops supplies.

Which did you go for? Here’s my reasoning:

a) works, if you’re talking about one specific Roman, and if he built (or was responsible for building) the bridges and roads being used by the many troops.
b) works, if you have lots of Romans and lots of troops. This is the ‘correct’ answer.
c) also works (imho), if you’re talking about one *specific* troop.
d) is an offence against punctuation, and should never be spoken of in polite company.

thoughts, comments, rants welcomed.