题目

. We should learn how to value what we have ______ it is too late. A. when                      B. after                       C. before           D. until 答案：C 用弹簧秤、量筒做测定某种物块密度实验，其主要过程和测得实验数据如图所示． 求：(1)物块浸没在液体中受到的浮力； (2)物块的密度．(量筒刻度单位为mL，g取10 N/kg)