Obstacles are the jobs that require knowledge other than that learned quite a lot. The need and the opportunity to learn more, even if this job is going to be important.
The position is a barrier that must be used in addition to other knowledge that students come to quite a lot. To learn more, and chances are, even if this position is important.
hazard is a job that must be used in addition to a study of knowledge, and quite another to do so, their own and the opportunity to learn more, even though it will be the main location.