The expression "A + B <--> C + D" represents a chemical equation. A chemical equation represents the "mixture" of molecules or elements that will react and form other molecules and elements, so we can say that A, B, C and D represent molecules or elements.
But not only that. In a chemical equation, the elements and molecules that are before the symbol "<-->" represent the reagents in the equation. Therefore, A and B are the reagents. The elements or molecules found after the symbol "<-->" are the products of the reaction. Therefore, C and D are the products.
Cigarette smoking decreases speed in track runner, due to the fact that their lungs have black spots. Making it harder for them to breathe. And harder to breathe causing slowing in speed.
