Which of the following is true concerning continental crust?
a. it does not contain radioactive elements.
b. it mostly contains heavier and denser minerals than the mantle.
c. it efficiently transfers heat from the mantle to the surface.
d. none of the above

The best answer would be letter D. There is no statement in the choices given that would be considered true concerning continental crust. The crust does not contain radioactive elements, and it is lighter and less dense than the mantle. Also, heat transfer goes from crust to mantle.

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