A．investors' strong preference for short-term relative to long-term bonds explains why yield curves typically slope downward
B．bonds of one maturity are not substitutes for bonds of other maturities; therefore, interest rates on bonds of different maturities do not move together over time
C．the interest rate for bonds of one maturity is determined by supply and demand for bonds of that maturity
D．all of the above
E．only b and c