Convection takes place
a. warm air is more dense than humid air.
b. warm and cold air have the same densityc. cold air is less dense than warm aird. cold air is more dense than warm air

The correct answer is:  [D]:  "Cold air is denser than warm air" .

If a particular resource is not useful or needed then the price would be considerably low. once that resource is needed the price or value will raise depending on how useful or needed it is especially if the resource is low in quantity, the consumers would pay whatever they have to, to get that material.