—I saw Dave in the lift this morning. —Really? He _______ around here for a long time. A. won’t be seen B. wasn't seen C. hasn't been seen D. hadn’t been seen 答案:D 解析: 略13.关于密度公式 ρ=$\frac{m}{v}$和压强公式p=$\frac{F}{S}$的比较,下列几种观点正确的是( )A.密度随着物体的质量改变而改变,压强也随着压力的改变而改变B.密度随着物体的体积改变而改变,压强也随着受力的改变而改变C.密度不会随着物体的质量改变而改变,压强也不会随着压力的改变而改变D.密度不会随着物体的质量和体积的改变而改变,压强却要随着压力和受力面积的改变可能要发生改变