For convection to occur,
a. dark objects are required to absorb energy.
b. substances must be in contact with each other.
c. particles must be able to flow within a substance.
d. heat must transfer from an area of less energy to more energy.

Answertraveling around a circular trackexplanation"traveling around a circular track" is the best answer as you could be traveling around a circular track at a constant speed. your velocity would therefore have the same magnitude but will always be constantly direction. since acceleration is the rate of change of velocity, the rate of change will only involve the direction of velocity and not the magnitude.wrong choices: "an object at rest" does not accelerate at all"increasing speed while traveling around a curve" means that the acceleration will not be constant due to only change in direction; it will be constant due to both magnitude and direction"an object in free fall" does not necessarily change direction 

Supply is the markets term for selling a specific quantity of products at various prices. the law of supply and demand is the one which affect the pricing and also the quantity the market is able to produce or make available.