题目

--- I wish I would succeed in the competition． --- Yeah, I understand how you feel．But ____hard?        A．did you work               B．have you worked         C．had you worked     D．do you work 答案：B 将电阻Rl与R2并联后接在电源两端时，通过它们的电流之比为3∶2，则它们的电阻之比为_________．若把它们串联起来接在电源两端，那么通过它们的电流之比为_________，电压之比为_________．