∵AB是半⊙O的直径，∴∠C=90°，设AC=4x，则BC=3x，∴AB= AC2+BC2 =5x，∴5x=10，∴x=2，∴AC=4x=8，∵OD⊥BC，∴OD∥AC，而O点为AB的中点，∴OD= 1 2 AC=4．故选B．
