A. in front of B. over C. beside D. behind E. between
2656Fe2+微粒中的质子数是 ____ ,中子数是 ____ ,核外电子数是____
如图所示,电源电压不变,R1=20Ω,只闭合开关S时电流表的示数是0.15A,再闭合开关S1时电流表示数是0.21A.求: