Why do Volcanoes only occur at destructive and constructive plate boundaries

They have different genesis, on destructive plate boundaries were an ocean plate subducts bellow continental plate, some magma get trapped and become highly pressurized and find a weak point to surface.Constructive plate boundaries receive a hot uprising current from the mantle bellow (mantle convection cell) and plates are pushed away one from the other generating a rift which is not exactly a volcano but magma is flowing intermitently to surface.Some volcanoes appears in middle of a plate (Hawai belt) due to hot points produced by local mantle cells which weakens the crust allowing magma to flow to surface, the plate have its own movement so the uprising current make a scare on it (the belt).

A,b,c is the answer to the question

Answer your driving test ;