$$R = \frac{{{R_1}{R_2}}}{{{R_1} + {R_2}}}$$  the formula represent?

A. an inverter B. a ups C. a step up transformer D. a step down transformer

A. 40 Ω B. 20 Ω C. 60 Ω D. 800 Ω

A. charge flow B. voltage flow C. water flow D. air flow

A. conduct electricity at lower temperature B. offer high resistance to the flow of current C. offer not resistance to the flow of current D. conduct electricity at high temperature