The inner core is solid because it is made of very dense, or heavy, materials - like iron and nickel. Even though it is very hot, these materials don't "melt" very easily, so they stay solid. Also, pressure due to the weight of the mantle and crust, atoms are forced together as solid.
I'll see if they have been so long ago that you can get the idea is that it
Coastal plains and Interior plains is in elevation. Coastal plains have low elevation and low relief. Interior plains have varied elevation and low relief. Coastal plains also lie along the shore while Interior plains are surrounded by other hopefully this helps
