1. The non-responders are not responding for similar reasons.
2. Those reasons are somehow related to their opinions.
Both are clearly false. You cannot assume anything about their opinions from the fact that they didn't respond. All you have is their (non)action - which could have been caused by being ill, busy, distracted by personal issues, absent, drunk, or for any other number of reasons.
None of the answers are correct. If you wanted representative data you would have to:
1. Confirm with a different sample but also...
2. Use multiple different channels and methods (phone, in-person survey on campus, maybe an email survey with some kind of benefit to encourage participation, etc) to try to eliminate the non-responders who have personally valid but statistically irrelevant reasons (away, ill, etc).