During which period is the substance changing state? A. During the time labeled C, when the temperature increases
sharply.
B. During the time labeled E, when the temperature increases for a
third time.
C. During the time labeled A, when the temperature first starts to
increase.
D. During the time labeled D, when the temperature stays the same.
SIDANT

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: During the time labeled D, when the temperature stays the same is correct.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.