Choose the pair of statements that are true with regards to the naming and sign convention for the viscous stresses that includes two subscripts.
a) The first subscript refers to the direction of the surface normal and the second subscript to the coordinate direction the stress points in. When the surface normal points in the negative coordinate direction, positive stresses points in the negative coordinate direction
b) The first subscript refers to the coordinate direction the stress points in and the second subscript refers to the direction of the surface normal. When the surface normal points in the negative coordinate direction, positive stresses point in the negative coordinate direction
c) The first subscript refers to the direction of the surface normal and the second subscript to the coordinate direction the stress points in. When the surface normal points in the negative coordinate direction, negative stresses point in the negative coordinate direction
d) The first subscript refers to the coordinate direction the stress points in and the second subscript refers to the direction of the surface normal. When the surface normal points in the negative coordinate direction, negative stresses point in the negative coordinate direction