Which of the following instances is an example of deterrence?

a. A state develops a series of fortifications that make it impractical for neighbors to invade it.
b. A state establishes a second-strike bomber fleet capable of destroying its neighbor's industrial infrastructure.
c. A state establishes a first-strike bomber fleet that results in its neighbor doing the same.
d. A state launches a pre-emptive strike against a neighbor that was mobilizing to invade it.
e. A state launches a retaliatory strike against a neighbor that has invaded its territory.

Answer: b