Salt is a substance because a substance is a material that has constant composition regardless of its source. Salt is the only option that matches this description
Scientists use theories to explain these things
Answer:
Average speed is 60 km/hour
Explanation:
When we need to calculate average speed, we use this equation:

Where:
position at the beginning
at the end


Then: 

Finally V = 60 km/hour
That's the definition of the property called "density".
Conduction and <span>convection it involves particles.</span>