The change of volume per unit time is __________

• A. \(\frac{∂∆V}{∂z}=-V_0 ∆σ’\)
• B. \(\frac{∂∆V}{∂z}=-a_v V_0 ∆σ’\)
• C. \(\frac{∂∆V}{∂z}=-m_v \frac{∂∆σ’}{∂t}\)
• D. \(\frac{∂∆V}{∂z}=-m_v dxdydz \frac{∂∆σ’}{∂t}\)