A. The pressure is lower as you gain altitude because there are more molecules of oxygen and nitrogen. B. The pressure is lower as you gain altitude because there are less molecules of oxygen and nitrogen. C. The pressure is the same because there is no change in the number of molecules.
The price of a cockatiel depends on a few factors. Firstly the price depends on whether or not the bird is tamed. For a tamed cockatiel it would cost around $80 - $120 and a untamed cockatiel would cost $15-$30 (AUS) Also the colouring of the cockatiel plays a part in the cost of the bird, as more rare colours (such as lutino or pearl cinnamon) may increase the cost of the cockatiel.
Chinchillas vary in price depending on their colour, quality and where you buy them. A Standard Grey, just for a example, might cost Â£30-Â£60 from a private seller and Â£100+ in a pet shop. These are just approximate prices as they vary so much. Different colours and 'show' chinchillas can cost hundred of pounds.