Which of the answer correctly explains how the atp molecule differs in function from the adp molecule?

A: ADP requires energy to make ATP, even though the net gain by the release of energy from ATP is > energy required to make ATP from ADP. ATP may be considered potential energy until it is actually used as an energy source 

c: ADP has 2 phosphates and to get energy from this, more energy must be put into the system to remove a phosphate than the energy released upon the removal of the phosphate. 
ATP has 3 phosphates and to get energy from this, the 3rd phosphate can be easily removed to give this energy. ADP could be considered potential energy in that it only requires the attachment of a phosphate group to make ATP. 

to me, there isn't really a clear answer for a or c 
if i had to choose, i would go with C