This question already has an answer here:
- “User accounts” or “users account” 4 answers
I'm sorry I couldn't word that better, the following example will hopefully clarify:
As you can see, the teacher refered to the bank of words as "word bank", as opposed to "words bank". Which one is correct? If "words bank" is the only correct usage then wouldn't it be fair to say that the exercise was resolved perfectly?