Compare how heat flows during exothermic reactions and endothermic reactions. Explain what happens to the thermal energy of the surroundings in each case

Firstly, the water in the dams hold gravitational potential energy as they are above ground level. the gravitational potential energy is then transferred to kinetic energy as it falls, spinning the turbinethe turbine then spins the generator, which is a large magnet surrounded by metal coils. by doing this, the generator generates electricity. in this action, the energy transfer taken place is kinetic to electric energy

Combustion is the act or process of burning. therefore, i believe the answer is  a: burning wood. 

Naoh + hbr -> nabr (salt) + h2o ca (oh)2 + 2hbr - - - > ca (br)2 + 2h2o