有两个定值电阻R1和R2,R1标有“10Ω 1A”、R2标有“20Ω 0.5A”,字样,把它们串联起来接入电路,则两端所允许加的最大电压是[ ]A.30V B.20V C.15V D.10 V答案:C来源:搜全贸