For a function to be invertible, it must pass the "horizontal line test." That is, there can be no horizontal line that will intersect the graph in more than one place. The bottom two graphs fail this test, so do not represent invertible functions.
Let be a sequence that converges to . This means for any , there is some such that for all . From this inequality we see that , so it follows that .
On the other hand, let be a sequence that converges to 0. This means for all large enough , and we get the simpler inequality for free, , so it follows that .