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.
Ok so Cockatiels come in many color mutations. Lutino, cinnamon, pied, pearl etc. It's just the way their feathers are colored. Same bird, different feathers. Some pet stores charge more for some mutations than others. Here in Florida, I got my cockatiel in a pet store for 99.99 and he was the same price as a pearl and a lutino, so it depends on the store. A good breeder will usually charge you between 40 and 60 dollars for a hand raised baby.