二、請分別說明反距離加權(inverse distance weighting, IDW)、雲形線(spline)及克利金(kriging)三種空間內插法(spatial interpolation)的基本原理，並分別舉例說明其適用情況。(25%)

4. A bond's rating provides a guide to its price. As we write this in 2009, Aaa bonds yield $ 5.45\% $ and Baa bonds yield $ 8.67\% $. If some bad news causes a $ 10\% $ five year bond to be unexpectedly downrated from Aaa to Baa, what would be the effect on the bond price? (Assure annual coupons with face value $1,000) (10分)