The sensitivity factor K of the strain gauge refers to （应变片的灵敏度系数是指____）

A、resistance value (电阻值);
B、Relative resistance change of strain gauge caused by unit strain (单位应变引起的应变片相对电阻变化);
C、The relative change of the cross-sectional area of the strain gauge wire (应变片导线截面积的相对变化);
D、Relative change of resistance value of strain gauge wire (应变片电阻值的相对变化)