Consider a steady flow through a horizontal divergent channel, as shown in the figure, with supersonic flow at the inlet. The direction of flow is from left to right.

Pressure at location $B$ is observed to be higher than that at an upstream location $A$. Which among the following options can be the reason?

1. Since volume flow rate is constant, velocity at $B$ is lower than velocity at $A$
2. Normal shock
3. Viscous effect
4. Boundary layer separation