Drives you nuts. There is nothing in the statement that could make any difference as to whether it is singular or plural. Yet you get marked wrong for such a petty thing.
It's both pedantic and absurd. Yet you wrote this a year ago and nothing has been corrected. Is this an abandoned course?
Precisely. I suspect it's because the people (who've put in a lot of hard work) doing this course learnt their English in America. "Happy Holidays" is something one hears in the USA in December to avoid favouring any particular religion. But in a general sense, outside the USA, one could hear 'happy holiday'.
Perhaps 'happy holiday' should be accepted, but I've never once heard anyone say that. Yes 'happy holidays' is only used around Christmas, but at least some people actually use that phrase. I don't think there's really any equivalent phrase to 节日快乐 that could be used at any festival in English.
Chalking their mistakes up to nepotism seems a wild conclusion to make. Besides that, making a program that allows all accepted ways to say the same thing in any natural language is very difficult even if they were all native English speakers. Try coding for the one thousand ways to say "got it/understood".
The 'r' in most any Asian language is very dissimilar to English or 'average Indo-European'. The second word is 日 with PinYin 'rì' with a relaxed 'i' that is used only to indicate the falling tone; that being the reason the vowel sounds more as an /u/.
Important to note that particular words in Chinese show a PinYin character yet only the consonant initial should really be said: This is one reason learners find difficulty in being understood due to belief that the written PinYin, a system developed by German priests, fits well into relational expectations between sounds and letters. The finals 'i' and 'e' tend to have this feature hence 吃 and 气 are far from rhythms despite the same literary symbol 'i' and first tone.