A boy was racing a bear cub to the top of a tree. The boy, with a mass of 30 kg, would grab hold of a branch with his hands and use his arms to pull himself up with 500 newtons a distance
of 0.5 meters. He would then lift his legs over the branch and then use his legs to push against
the branch with 800 newtons to lift himself another 0.5 meters.
How much work did the boy do with his arms to lift himself over one branch?

The answer to the question is (a)   

Answer;

hot magma rises and pushes up toward the lithosphere, and as it cools it cycles back down toward the core to be heated again.

explanation; convection currents are the circular pathways taken by a fluid that enhances heat transfer by the process known as convection which occurs in fluids.convection currents transfer heat from one place to another by mass motion of a fluid. earth mantle is in a semi-liquid state and behaves like any other fluid, rising up from the bottom of the mantle after becoming hotter and less dense from heat of the earth's core, as the rock loses heat into the earth's crust it becomes cooler and more dense, then sinking back down to the core, then repeating the cycle.