Which statement describes the appearance of a temperature-vs.-time graph? a horizontal line shows that the temperature increases at a constant rate over time. a vertical line shows that the temperature decreases at a constant rate over time. horizontal lines where the temperature is constant during phase changes connect upward-sloping lines where the temperature increases. horizontal lines where the temperature increases are connected by upward-sloping lines where the temperature is constant for each phase.

Horizontal lines where the temperature is constant during phase changes connect upward-sloping lines where the temperature increases.  

Step-by-step explanation:

The temperature is constant during a phase change (melting or boiling), so the graph is a horizontal line.

The temperature increases when a single phase (solid, liquid, or gas) is heated, so the graph for these changes has an upward slope.

A is wrong. A horizontal line shows a constant temperature.

B is wrong. There are no vertical lines in a heating curve.

D is wrong. A horizontal line does correspond to an increase in temperature.