Which of the following statements concerning mortgage$-$backed securities are correct?

I. They are secured by a pool of residential mortgages.

II$.$ A portion of the income stream is a non$-$taxable return of capital.

III. They are backed by the full faith and credit of the government.

IV$.$ Their maturity depends on prepayments of the mortgages in the pool.

A$.$ I and III only

B$.$ I and IV only

C$.$ II$,$ III and IV only

D$.$ I, II and IV only