2) they add the insulation to better the durability or the earbuds
The best logical answer is A
When sphere A and B are brought in contact and separated, charge on each sphere becomes [2x10^-6 + (-4x10^-6)]/ 2 = -1x10^-6 C.
That is, charge is equally separated and is the average of charges on both spheres. The reason behind equal charge on both spheres after separation is, when they are kept in contact, their potential difference becomes same.
Yes it is valid all the times under the consideration of acceleration due to gravity .it is not valid on space where there is no influence of gravity