The united arab emirates was able to use over 600 billion gallons of water in 2004 by a. a large port system to trade oil for water. b. a large system of irrigation pipes. c. a large number of desalination plants. d. a large number of dams to store water.

The correct option is C.
The United Arab Emirate region was afflicted with scarcity of water, therefore they make use of ocean desalination method to produce huge amount of fresh water from ocean water on a daily basis. It was on record that one of the desalination plant in Dubai was making half a billion gallons of fresh water from ocean water on a a daily basis.

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