Read the passage below and select the sentence number that best expresses the main idea.(1) In 1980 Mount St. Helens in Washington state erupted, killing nearly a hundred people and leveling thousands of acres of timber. (2) By contrast, most Hawaiian volcanoes never erupt; instead, they simply emit flowing lava. (3) Whether a volcano erupts or simply emits lava depends on the composition of its molten rock, which comes to the surface as lava. (4) Volcanoes like Mount St. Helens have quick-cooling lava. (5) As a result, the lava forms a dome that plugs the volcano’s hole. (6) Eventually the pressure becomes so intense that a tremendous explosion blows off the lava dome and spews out a massive cloud of ash. (7) Instead of hardening soon after it reaches air, the lava of most Hawaiian volcanoes flows down the cone’s sides until it encounters an obstacle or slowly cools into a solid. (8) Because the lava never plugs the volcano, the volcano never erupts.