In structural engineering, what does the term 'shear lag' refer to$?$

$-$ A$)$ The reduction in effective tension across a bolted connection

$-$ B$)$ The uneven distribution of shear stress in a beam

$-$ C$)$ The tendency of a structural member to buckle under compression

$-$ D$)$ The reduction in effective cross$-$sectional area resisting shear forces in a built$-$up member