The Magnetic Field of a Plane-Polarized Electromagnetic Wave Moving in the Z-Direction

Multiple Choice

The magnetic field of a plane-polarized electromagnetic wave moving in the z-direction is given by
$B = 1.2 \times 10 ^ { - 6 } \sin \left[ 2 \pi \left( \frac { z } { 240 } - \frac { t \times 10 ^ { 7 } } { 8 } \right) \right]$
in SI units.What is the speed of the EM wave?

A) 3* 10⁸ m/s
B) 100 m/s
C) 10⁶ m/s
D) 2 * 10⁷ m/s
E) 2 * 10⁸ m/s