I have found this link
Which apparently ask same question like me right now. But, the answers don't provide me information to solve the problem. Once again, I have read many times about what was discussed in the comments, but still I don't understand. This one: @JAEMTO Ok. Suppose there is an edge AB which is not a bridge. Then after removing it there is a path from A to B. That path cannot involve the edge AB because you have just removed it. So how does that give you a contradiction?
The theorem may be simple to solve, but somehow I can't proceed how to prove it.
Please help me. Regards.