[单选题]
An electromagnetic wave is xo8q c0)x; 4fp/f1jk7ba qw cwtraveling xi,4c 60uy 3b3eep0o0gfkkyv1 q q e+vto the east. At one instant at a given point its 1e0y quv 0ibp k goy f3ekq,63e+cv04x$E$ vector points straight up. What is the direction of its $B$ vector?
