How are boundaries between river systems and natural vegetation regions different?

Rivers are divided by watershed boundaries, lines that follow the hirght og the loand and the water drians off in the opposite directions. In nature it is virtually impossible to see where oneregion of vegetation starts and ends. Natural vegetation regions gradually change through transition zones.

A,) cause a & b are equal & both gives 180°

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