We don't know that at all. The 3rd law says that the REaction is opposite and EQUAL to the action. We don't know where that "twice as much" comes from.
20N..........................
Answer:
The answer is "Choice C ".
Explanation:
The relationship between the E and V can be defined as follows:
![\to E= -\Delta V](https://tex.z-dn.net/?f=%5Cto%20E%3D%20-%5CDelta%20V)
Let,
![\to E= \frac{\delta V}{\delta x}](https://tex.z-dn.net/?f=%5Cto%20E%3D%20%5Cfrac%7B%5Cdelta%20V%7D%7B%5Cdelta%20x%7D)
When E=0
![\to \frac{\delta V}{\delta x}=0](https://tex.z-dn.net/?f=%5Cto%20%20%20%5Cfrac%7B%5Cdelta%20V%7D%7B%5Cdelta%20x%7D%3D0)
v is a constant value
Therefore, In the electric potential in a region is a constant value then the electric-field must be into zero that is everywhere in the given region, that's why in this question the "choice c" is correct.