Why does economy doesn't work with a strong middle class?

1) earth's crust is made up of relatively rigid plates that ride atop earth's hot, semiliquid mantle. (2) the plates are called tectonic because they're in constant motion. (3) they can move because earth's mantle is a very hot and semiliquid fluid called magma. (4) volcanoes are a result of magma rising up or erupting through a plate, particularly where plate boundaries are moving against each other. (5) also, when plates slide against each other, causing friction along adjacent plate boundaries, earthquakes frequently occur. (6) therefore, we often find volcanoes and earthquakes along plate boundaries. (7) plates may also collide. (8) when that happens, mountain ranges are formed. (9) for example, the collision of the plate carrying the indian subcontinent created the himalayan mountains when it collided with the asian plate. 2. which sentence in this passage introduces a new topic that might begin a new paragraph? a. sentence 5 b. sentence 4 c. sentence 7 d. sentence 6