Having re-read through the thread, I defer to Angelika's comments:
In my 60+ years or reading, writing and speaking English, including provision of advice as a mentor, though not (yet) as a teacher, I personally wouldn't have used the plural form of the noun. As Angelika says, it appears that both forms are available, but the singular can be and is used as the plural.
In my career in the Oil & Gas industry, I have, however, regularly (almost constantly) been exposed to the US variant of the English language - and we all have to be very careful in our advice in such forums as a result.
We are, unfortunately, as someone once said: "2 countries separated by a common language!".
PS In this context, I have just failed to obtain 100% on one of the tests, as I believed "worthful" to be a valid "English" word. It apparently is so in American English (Ref: worthful - definition of worthful by the Free Online Dictionary, Thesaurus and Encyclopedia.
) but not in OED!
Angelika: Maybe the testing should either declare that it is based UK rather than American English or the tests adjusted to allow for both?