Help I'm desperate and need help ASAP I will give brainliest to whoever gets it correct. Select the best answer from the choices below. Standing waves are formed by:

a
A wave hitting the surface of an object and bouncing back
b
Light moving from one medium to another
c
Two or more identical waves moving in opposite directions
d
Waves moving around an obstacle

Four point charges each having charge q are located at the corners of a square having sides of length a. find symbolic expressions for the following. (a) the total electric potential at the center of the square due to the four charges (use any variable or symbol stated above along with the following as necessary: ke.) vtotal= (b) the work required to bring a fifth charge p from infinity to the center of the square (use any variable or symbol stated above along with the following as necessary: w=