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)