The united states highway system is a in the united states numbered within a nationwide grid.

The United States Highway System is a network of roads and more specifically a network of strategic highways that provide communication between the cities and states.

C. naturally in the environment

So  if  the sum of the enthalpies of the reactants is greater than the products, the  reaction  will be  exothermic.  if  the products side has a larger enthalpy, the  reaction  is  endothermic. you may wonder why  endothermic reactions, which soak up energy or enthalpy from the environment, even happen