Alongshore current flowing southwards along a coast will primarily transport sediments

Southwards parallel to the shoreline.

A geological course that is made up of the movement of sediments like silt, clay, shingle and sandd within a coast parallel to the shoreline, which is reliant on oblique approaching wind direction is called longshore drift. Oblique incoming wind restraints water along the coast, and so creates a water current which moves parallel to the coast.

i think the answer would be d

a=c and then d= c because of the band so you get ac/dc and you add them together you get sound waves