The given figure shows the heating curve of a substance.
Straight parts ab and cd of the curve show that
ANo heating is done during this period.
BHeat energy is used to overcome the forces of
attraction between the particles.
CThermometer showed some error during this period.
DEnergy of particles of the substance is zero during
this period.
This is either a wrong question or if there is some trick then surely not class 6th level question.