将电阻为R1=3Ω的导体与电阻为R2的导体串联后总电阻为9Ω,则R1与R2并联后总电阻是
A:6Ω
B:4Ω
C:2Ω
D:1Ω
有一瓶无色溶液,可能含有Ba2+、NH4+、Cl﹣、CO32﹣等离子,为验证其中所含的离子,做如下实验.
When he ___ the bus , the poor man realized he had left his bag on it.
科学探究成功的关键是( )