A trolley begins from rest at X. It moves down to Y and finally stops at Z. Which row could give a correct summary of the energy changes in this process?
- AX to Y: p.e. $\rightarrow$ k.e.; Y to Z: k.e. $\rightarrow$ p.e.
- BX to Y: p.e. $\rightarrow$ k.e.; Y to Z: k.e. $\rightarrow$ p.e. + heat
- CX to Y: p.e. $\rightarrow$ k.e. + heat; Y to Z: k.e. $\rightarrow$ p.e.
- DX to Y: p.e. $\rightarrow$ k.e. + heat; Y to Z: k.e. $\rightarrow$ p.e. + heat