In the next two questions, we consider the model of Vasicek (1977, "An Equilibrium Characterization of the Term Structure", Journal of Financial Economics), which is a standard model of the term structure of interest rates. The state variable is the short-term interest rate, which is assumed to evolve as drt=a[brt]dt+rdzt where z is a Brownian motion, and the parameters a,b and r are constant. It can be shown that in this model, the price at time t of a pure discount bond that pays $1 at time T has the form bt,T=exp[D(Tt)rt+C(Tt)] where C and D are two functions of the bond maturity. The expression of C is irrelevant here, and D is given by D(u)=a1exp[au] We write the dynamics of the bond price as bt,Tdbt,T=b,tdt+b,tdzt What is the expression for b,t, which measures the instantaneous exposure of the pure discount bond to changes in the Brownian motion? b,t=D(Tt)r b,t=D(Tt)r b,t=C(Tt)r b,t=21C(Tt)r