It has to be the last one because whenever lights are turned on it decreases because all lights are on at the same time. It's good to just have one light on. It doesn't use as much electricity.
Answer:
C). 
Explanation:
As we know that capacitance of a given capacitor is

now we know that energy stored in the capacitor plates

here if all the dimensions of the capacitor plate is doubled
then in that case

here area becomes 4 times on doubling the radius and the distance between the plates also doubles
So new capacitance is now

so capacitance is doubled
now the final energy stored between the plates of capacitor is given as

so the final energy is


Wave Interference or Interference of wave