Answer:
Horizontal and pointing south.
Explanation:
The Poynting vector points in the direction of propagation (if the wave is a sinusoidal linearly polarized plane wave of fixed frequency, as it is in our case), and it's given by (all vectors):
[tex]S=E\times H[/tex]
where [tex]H[/tex] points in the same direction as [tex]B[/tex], the magnetic field. This means that [tex]E[/tex] must point south, because the vectorial product of a vector pointing south by a vector pointing up gives a vector pointing west. This can be deduced by the right-hand rule: if our index finger is [tex]E[/tex] and our middle finger is [tex]H[/tex], then the thumb is [tex]S[/tex].
The direction of the electric field vector of this wave is vertical and pointing down.
This is defined as the electric force per unit charge. The direction is determined by pointing the fingers of the right hand in the direction of the electric field and curling them toward the direction of the magnetic field.
We will notice that the thumb will point downward in a vertical manner which was why option A was chosen as the most appropriate choice.
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