High diagram shows the valence electron pattern for a carbon atom

The right answer is A.

The carbon having 6 electrons adopts an electronic configuration in the ground state 1s2 2s2 2p2. It has four electrons on its valence layer, which allows it to form four covalent bonds, including "sigma" (first bond with one atom) or "pi" type bonds (second or third bond). "Pi" type links are always accompanied by a "sigma" type link. The recovery of electronic functions in a "pi" link is weaker. These links are therefore less "solid".

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.