[D to the iddy]

As mentioned above, I would try to see if there are overlapping topics that you can study and get enough knowledge in your brain to answer two of the questions. Having 10 different topics for a University course makes me think that some of them should overlap even just a little. Finding these areas of overlap can help eliminate some study time.

As for whether studying 7 questions is worth it, that's a personal choice. It's a gamble that you're either willing or not willing to take. If you're doing good in the course and don't really care, then only study 6 or 7 and take the risk. If you need to do good on the test to get a good mark in the course, then study all 10.