Q: 20 (NDA-II/2008)

A solid is melted (below the melting point) and allowed to cool down at normal condition. Its variation of temperature as a function of time is as shown in the figure given below. What is the reason for the plateau (flat position) in the central region of the cooling curve as shown in the figure?
question_subject: 
Science
question_year: 
2008
question_exam: 
NDA-II
stats: 
0,88,424,304,112,88,8