题目

Usually it cannot (get out) because the outside of the earth is too thick and strong. But in some places the outside of the earth is ____ and weak.     A. thin     B. thick      C. flat     D. water 答案：A 如图所示，电压表测的是     两端的电压，若电源电压是6V，电压表的示数是2V，则L2两端的电压是     V．