I have read that you should always put a positive spin on your answers. For instance, if you say your greatest obstacle was finishing you degree while having to work part time. You can then say you really had to learn to budget your time and set priorities so you could get all the important things done and still earn a living. Or, you could say your obstacle was not leving close to your school and it was hard for you to get to the library so you had to get really good at using the internet for research and finding the information you needed.

Good luck.