What type of gases holds in heat and keeps the earth at a moderate temperature?
primary gases
ozone layer
trace gases
greenhouse gases

The answer is Greenhouse gases

Greenhouse gases absorbs and emit radiation within thermal infrared range . This process create a condition which commonly known as the greenhouse effects, keeping the earth at a moderate temperature.

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

Well, a and b together would be negative. b alone would also be negative.