Legacy applications, particularly those which control electrical and electronic components of critical infrastructures, rarely had security built into the software itself. For these applications, a "bolt security on$"$ strategy is required to reduce the impacts of cyberattacks. Which of the following is the best strategy for "bolting on$"$ a layer of security for these vulnerable legacy applications?

Question $3$ options:

Insert an air gap to isolate the vulnerable software from the network.

Recoding the software application to add controls.

Adding perimeter defenses such as firewalls to limit network access.

Add two$-$factor authentication to prevent hackers from gaining access to the application.