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

A mobile phone charger is

A. an inverter

B. a ups

C. a step up transformer

D. a step down transformer

What will be the total resistance of the three 20-Ω coils connected in series?

A. 40 Ω

B. 20 Ω

C. 60 Ω

D. 800 Ω

AC and DC are the form of?

A. charge flow

B. voltage flow

C. water flow

D. air flow

Superconductors are substances which

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