Me answer these science questions plz plzzz questions 18,20,21

Idk 18 but here is 20

conductor-a substance that readily conducts
insulator-a substance that does not readily allow the passage of heat or sound
ammeter-an instrument for measuring electric current in amperes.
voltmeter- an instrument for measuring electric potential in volts
current-a flow of electricity which results from the ordered directional movement of electrically charged particles.
potential difference-the difference of electrical potential between two points.
ohmic resistor- resistors that have a constant resistance
non-ohmic resistor- a resistor that's resistance increases as the current increase
power-the capacity or ability to direct or influence the behavior of others or the course of events.