Water is heated by submerging 50-mm-diameter, thin-walled copper tubes in a tank and passing hot combustion gases ( 750 k) through the tubes. to enhance heat transfer to the water, four straight fins of uniform cross section, which form a cross, are inserted in each tube. the fins are 5 mm thick and are also made of copper ( 400 w/m·k).

r=rho l/a where a is the area, l is the length and rho is resistivity so the greater the area the lesser the resistance thats why the earth pin is thicker bcos current follow the shortest path, so less resistive .

co2reflects heat radiation back to earth buning coal and oil emits co2 globe warms

there are no contact forces mentioned on the list of choices that you've provided.

amplitude

hope this