(i added the image)

the image below is a cross-section through the earth's interior. tectonic plates of the lithosphere are shown in grey at the earth's surface. the earth's mantle is shown in red-orange. black arrows in the lithosphere show the directions in which tectonic plates move. red arrows in the mantle represent convection currents. the part of the diagram labeled as "ridge" represents a mid-ocean ridge, where two tectonic plates move away from each other. the part of the diagram labeled as "trench" marks an ocean trench, a location beneath which one plate plunges under another. according to this diagram, convection currents begin to move beneath mid-ocean ridges and begin to move beneath trenches.

a.
upward; upward
b.
downward; upward
c.
upward; downward
d.
downward; downward