For an Electromagnetic Wave in Free Space Having an Electric $E$

For an electromagnetic wave in free space having an electric field of amplitude $E$ and a magnetic field of amplitude $B$ , the ratio of $B / E$ is equal to

A) $1 / c$
B) $c$
C) $1 / c ^ { 2 }$
D) $c ^ { 2 }$
E) $\sqrt { c }$