Occur at underwater subduction zones

Volcanoes occur at underwater subduction zones. In fact, volcanoes form at most if not all types of subduction zones. Subduction zones are regions where tectonic plates collide and one plate bends and slides underneath the other plate. This bent plate reaches the mantle of the Earth, putting more pressure on the hot materials inside it.

The correct answer is:   [b]:   "distortion" . 

2pi more meters because you find the circumference which is 6pi and 4pi and subtract it and you get 2pi