What is the ratio of the transfer function Io/Is?

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