Read the excerpt from The Code Book. Despite the enormous strength of RSA and other modern ciphers, cryptanalysts are still able to play a valuable role in intelligence gathering. Their success is demonstrated by the fact that cryptanalysts are in greater demand than ever before—the NSA is still the world's largest employer of mathematicians.

What is the central idea of this paragraph?

Many math experts work for the National Security Agency.
The RSA computer system makes our country more secure.
Skilled people are still important national security workers.
Computers are better than people at keeping the nation safe.