Problem associated or encountered when using River​

Depending what your using the river for it could be the following:
If you use it for drinking then you can run into contaminated/dirty water especially if the river is close to densely populated areas like city’s or factories as drinking this water without taking the proper precautions can get you very sick and could possibly lead to death
If you use the river for travel the speed of the current and objects that happen to be in there like sharp rock etc. this could be thing that could drown you or injure you and referring to the dirty water again if that gets into the wound then it could cause infection
If the river is used for anything else you have to remember that you aren’t the only one that needs water to survive so it can possibly a hot spot for dangerous animals and of course all of this is circumstantial

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

The correct answer is:   [b]:   "physical systems" .