It was not until he told me the news ________ I learned that Tom had won the prize in the 16th Asian

Games. [      ]

A. when  

B. which

C. if  

D. that

某实心正立方体金属块的质量是m，边长是a，则金属块的密度ρ=     （用m、a表示）。取6个这样的金属块，分别以下图的甲、乙两种方式放置在水平地面上，则甲、乙两种方式放置的金属块对水平地面的压力之比F_甲：F_乙=                  ，压强之比P_甲：P_乙=              。