Answer:
Volcanoes are formed at the divergent and convergent plate boundaries.
During a divergent plate motion, along the mid-oceanic ridge, seafloor spreading takes place. Here it is represented by location B. Due to this, the magma becomes extremely hot and exerts an upward pressure towards the seafloor. As a result of which the ocean floor slowly rises up forming sea volcanoes. The eruption takes place and the lava forms and deposits on the seafloor, near and along the mid-oceanic ridge. As this region undergoes continuous spreading, so the crust comprised of these rocks slowly moves away from the ridge.