Chapter 23.2, Problem 2EYU

What is the angle θ in the definition of magnetic flux  Φ = BA cos θ through a surface? (a) The angle between $\stackrel{\to }{\text{B}}$ and the plane of the surface; (b) the angle between $\stackrel{\to }{\text{B}}$ and the x axis; (c) the angle between $\stackrel{\to }{\text{B}}$ and the normal to the surface, (d) the angle between the surface and the x axis.