You observe the 18-month and 2-year zero coupon rates for U.S. Treasury securities are 1.25% p.a. and 1.35%, p.a., respectively. Assuming arbitrage-free markets and no friction costs, the implied 6-month rate in 18 months' time should be closest to: A. 1.15% B. 1.28% C. 1.32% D. 1.65%