A $50 million pool of mortgages with a weighted average coupon of 5.2% net of fees is securitized into a CMO. This CMO has 4 tranches as listed below with the coupon rates. Tranche Rating Amount (US$) Coupon Rate AAA-Rated $35 million 2.50% A-Rated h $ 5 million 4.10% BB-Rated $ 5 million 7.10% Unrated $ 5 million ??? What is the coupon rate of the unrated tranche? A. 13.83% B. 16.31% C. 18.74% D. 23.30%