No. That's the whole point of your homework/test question. We are not going to do it for you. It is against the terms and conditions of this forum which you agreed to by signing up here (besides being morally and ethically wrong).
What is it that you want ? You have already been given the links for that, still you ask for an explanation. But I guess the links would be carrying an explanation itself but in the words of the author who developed the Abstract Classes and Interfaces themselves or in the words of the someone else who the authored trusted to be smart enough to be able to explain that. Now still do you want an exaplantion in the words from someone from among the forum - who would not be someone the author/developer trusted to do the job.
I am not saying that the explanation you would get from here would be wrong but my point is, that the explanation you would get from the links would almost be horse's mouth stuff.
yes it might happen that the author/developer might be going through this forum looking for such untidy posts - as you say the chanes are slim - and in that case the poster might be able to get a 100% accurate explanation but the master links would certainly not carry wrong information and that my whole point which is summed up in the last statement.