Why is carbon is considered tetravalent?

why is carbon is considered tetravalent?
since it has four electrons in its outermost orbital, it needs four more electrons to complete its octet.
since it has two electrons in its outermost orbital, it needs four more electrons to complete its octet.
since it has four electrons its outermost orbital, it can donate four electrons to complete its octet.

What is binding energy and separation energy

Copper has free electrons per cubic meter. a 71.0-cm length of 12-gauge copper wire that is 2.05 mm in diameter carries 4.85 a of current. (a) how much time does it take for an electron to travel the length of the wire? (b) repeat part (a) for 6-gauge copper wire (diameter 4.12 mm) of the same length that carries the same current. (c) generally speaking, how does changing the diameter of a wire that carries a given amount of current affect the drift velocity of the electrons in the wire?

Imagine that someone pushes one marble toward a motionless marble. would there still be action-reaction forces involved in the collision? how might the marbles’ motions be changed? ?