Answer:
d) 0 V
Explanation:
It can be showed that the potential due to a point charge q, to a distance d from the charge, can be expressed as follows:

where k = 
As the potential is an scalar, and is linear with the charge, we can apply the superposition principle, which means that we can find the potential due to one of the charges, as if the other were not present.
By symmetry, all four charges are at the same distance from the center, so we can write the total potential, as follows:

where d, is the semi-diagonal of the square, that we can find applying Pythagorean theorem, as follows:

Replacing by the values in (1) we have:

which is equal to the option d).
If she read 41 pocketbooks this month then she read last month=41-13=28
Answer:
1/4 λ film
Explanation:
Let L = total path length then L = 2 t where t is film thickness
There will be a 180 deg phase change at the air-film interface but no
phase change at the film-air interface
L = n * wavelength / 2 where n = 1, 3, 5, 7 etc
(the total path L must be an odd number of 1/2 wavelengths)
Or t = n * wavelength / 4 (the film must be an odd number
of 1/4 wavelengths thick)
1/4 λ film satisfies this condition
Note: Find the missing diagram in the attachment section.