7.有一均質圓桿受力前長度100cm,直徑5cm,受拉力變形後,長度變為100.2cm,直徑變為4.999cm,則該圓桿材料之蒲松比(Poisionratio)為何?(A)0.1(B)–0.1(C)0.3(D)–0.3