Which idea is explicitly stated in this passage from every day heroes

Answer (d) the greenhouse effect is primarily caused by car and trucks;

The answer to the first one is barometer.

Answerinflux;

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.