Achild is on a pogo stick. at the top of her jump, kinetic energy (ke) = 0 j, gravitational potential energy (gpe) = 100 j, and the potential energy in the spring (spe) = 0 j. at which point in the jump is the kinetic energy the greatest?

Because just before the pogo stick contacts the ground the system has its greates velocity.  

2.) A  

Because they have the same mass and the same height.  

3.) D  

Because the velocity is 0 (KE is 0), The Height is 0 (GPE = 0) and the spring is compressed due to converting all the kinetic enrgy into SPE.