Two coins add up to thirty one of them is NOT a nickel. What are the coins???

1. crust 1 the metallic layer at the earth's center 2. inner core 3 the firm layer between the crust and the asthenosphere 3. lower mantle 2 the layer surrounding the inner core 4. asthenosphere 4 the semi-solid layer between the top layer and the upper mantle 5. upper mantle 5 the layer between the asthenosphere and lower mantle 6. top layer 6 the earth's outermost layer 7. outer core 7 the hottest part of the mantle

In which layer of the atmosphere are the particles so far apart that they can travel hundreds of kilometers without colliding with each other? a.exosphere b.troposphere c.mesosphere d.thermosphere

The thermal convection they drives plate motion is caused by