20. which of the following types of ground is subject to long-term settlement problems and should therefore be avoided for construction work? backfill over opencast mining sites; ground above longwall mining river terrace gravels; ground more than 30 metres above pillar and stall mining.

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

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The answer is d) rift