Which one of the following transfer functions represents the critically damped system?

• A. H1(s) = 1/s2 + 4s + 4
• B. H1(s) = 1/s2 + 3s + 4
• C. H1(s) = 1/s2 + 2s + 4
• D. H1(s) = 1/s2 + s + 4