you are wrong to mark the questions wrong if there are more than One way to say it in English.
A lot of the examples you are giving have more than one way of saying the sentence correctly .
Catherine is right. Plus, there is a feature meant for this: in such cases, if you hit "Report" then "My answer should be accepted", the course contributors receive a notice, and may very well add your solution. You then get an email to notify you, happened to me a number of times. Remember, course contributors are human beings, most of the time volunteers at that, they can't possibly think of everything.
Keep on your good (excellent, seeing your levels) work, and make the course better for everyone :)