Hi Shahd,
Q17:
What clues me into thinking about the main idea is that it is asking for the authors "primary" argument. The word "primary" makes me think about what their overall point and goal is.
Q20:
D - is the correct answer. It is actually suggesting a new argument because it is blaming the disparities on what happened during apartheid, instead of how changes implemented after apartheid are to blame for the disparities.
Hope this helps!