Which faces of an infinitesimal element (aligned with the cartesian coordinate system) can x_1 momentum be transported across?
A. The x_2 faces (i.e. the faces perpendicular to the x_2 direction but parallel to the x1 direction)
B. The x_3 faces (i.e. the faces perpendicular to the x_3 direction but parallel to the x_1 direction)
C. All of the faces
D. The x_1 faces (i.e. the faces perpendicular to the x_1 direction)
E. The x_2 and x_3 faces (i.e. the faces perpendicular to the x_2 and x_3 directions but parallel to the x_1 direction)