well, if you study hard adn know your stuff, it wont be hard. Remember to consult your unit outline carefully for the requirements of th assignment, usually they'll state what is needed fo each task in repect with the grade, so if you meet those requirements, than theyll tick it off and dish out the mark accordingly.
Also remeber that sometimes some assignemknts have different difficulty levels, for those who just want to pass and for those who wnat to achieve more, if choose the more difficult one if your aiming for more marks.
Some can study wiht less hours than other poeple, but it dpends on teh subject, i mean if its stats for instance, its really memorising formulas and aplying them to problems etc, but if its more like law or Arts and you need to be critical or write showing oyu understand and word it in an eloquent way, than thats whats needed.
You'll find out though after you finish your first semester and youll go phew, it wastn that bad after all. for first year, D is definately reachable if you focus, study and meet the criteria, so remember to read it thoroughly!