Here's why:

* Mid-ocean ridges are divergent plate boundaries: This means tectonic plates are moving apart, creating space for new oceanic crust to form.

* Subduction zones are convergent plate boundaries: This means tectonic plates are colliding, and one plate (the denser one) slides beneath the other.

* Mid-ocean ridges are where new crust is created: The rising magma from the mantle cools and solidifies, creating new ocean floor.

* Subduction zones are where crust is destroyed: The subducting plate sinks back into the mantle, eventually melting.

Therefore, subduction is the opposite process of what happens at a mid-ocean ridge.