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.

D - Silicon

This is because Silicon has four electrons as indicated by the group it's in. Chlorine would have 7, Magnesium would have 2, Phosphorous would have 5, and Sulfur would have 6.

Hope this helps!