Answer:

Explanation:
Phase changes have to happen at a specific temperature. During a phase change, the energy is used to make the change happen, instead of raising the temperature. 
On a graph, a constant slope, or in this case temperature, is a flat line. On this graph, the flat lines are at B and D. 
There is no answer including B, but there is for D. 
Choice D: <u>During the time labeled D, when the temperature stays the same</u> is correct.