#### Answer

If the link connecting node $\mathrm{F}$ to node $\mathrm{D}$ fails, then the paths ABFD and
AEFD will not work and their weights become "infinite." Of the two
paths remaining, path ABCD with weight 16 now becomes the shortest
path. No one link in the network will disconnect nodes $\mathrm{A}$ and D. We can
see that clearly by noting that the two paths $A B C D$ and AEFD do not
share any links in common. Therefore, if a link along one of these paths
fails, we can use the other path.