Why dont all the rotors of a quadrotor spin in the same dire

Why don’t all the rotors of a quadrotor spin in the same direction?

a. Spinning all rotors in the same direction does not allow the quadrotor to fly upside down.
b. Spinning all rotors in the same direction uses more battery power.
c. Spinning all rotors in the same direction will cause the robot to constantly rotate.

Solution

Spinning all rotors in the same direction will cause the robot to constantly rotate. This video justifies it: https://www.youtube.com/watch?v=ikQvolokf_Q

A quadrotor has four rotors all work together to produce upward thrust and each rotor lifts only 1/4 of the weight, so we can use less powerful and therefore cheaper motors. The quadcopter’s movement is controlled by varying the relative thrusts of each rotor.

These rotors are aligned in a square, two on opposite sides of the square rotate in clockwise direction and the other two rotate in the opposite direction.