Which feature of a heating curve indicates a change of state? an upward sloping line a downward sloping line a horizontal line a curved line

The heating curve

The curve is a graphical presentation of the phase of the transition of the substance as heat is added to it. The plateau shows us the mark in the changes in the heat of the substance. The rising temperature stays constant during the phase.

Hence the answer is a horizontal line.

The heating curve is opposite to the cooling curve and it shows the temperature of the substance when the cooling takes place. The heat is refed to the thermal energy and is shown along with the graph in the form a curve.Hence the constant line with no change is a horizontally flat curve.

Thus the option C is correct.

Learn more about the heating curve

link.

A "heating curve" is a graph that shows the temperature of the substance
against the amount of heat you put into it.

For most of the graph, as you'd expect, the temperature goes up as you
add heat, and it goes down as you take heat away.  BUT ... While the
substance is changing state, its temperature doesn't change even though
you're putting heat in or taking heat out.

So that part of the graph is a horizontal line. 

25.7km/h

Explanation:

Distance= 140km

Time = 3.5hours

Average speed= Distance/Time

= 140/3.5

=25.7km/h