The heating curve below represents a sample of
a substance starting as a solid below its melting
point and being heated over a period of time.
which statement describes the energy of the
particles in this sample during interval de?

(1) both potential energy and average kinetic
energy increase.
(2) both potential energy and average kinetic
energy decrease.
(3) potential energy increases and average
kinetic energy remains the same.
(4) potential energy remains the same and
average kinetic energy increases.