Answer:
The magma trapped beneath the subduction zone accumulates and finds its way through the crust.
Step-by-step explanation:
The volcanoes in A are product of the magma coming from the mantle into the ocean and cooling down, producing new seebed. The volcanoes in A are fissure volcanoes and remain active as long as the tectonic plate has margins under the ocean. The volcanoes in B are product of the oceanic crust melting beneath the continental crust.
The continental crust is floating over the oceanic crust (oceanic floor), so when there is a collision between oceanic floor and continent, the ocean floor goes beneath it. As the oceanic crust goes into the mantle, it melts and accumulates under the continental crust. This new material then starts to ascend by means of pressure and increase of temperature and escapes through the continental crust in the form of a volcano. The magmatic chamber erupts only when enough incandescent magma accumulates inside it.