I had at least 5, possibly 6. That I had so many suggests I was on the wrong track, as the question implied that only a couple would be needed. Still, the fact that you could make up your rules means virtually any CFG and hence (valid) parse tree automatically is "correct".
This however does lead me to another problem with this aspect of the course, in that it required a disturbingly high understanding and knowledge of English grammar. While this is perhaps fair enough if you're actually writing English language processors, it seems less relevant in an introductory course about the general topic of language processing. I therefore didn't study too carefully the specifics of the different English language rules, as it would have been inappropriate in an exam (which I remind you was also set in Afrikaans...). Needless to say, I got this assumption wrong too