Gas Exchange question

Gas exchange & mountaineering
Suppose you want to climb a mountain that is 19,000 feet tall. The air at that altitude
has a total pressure of about half an atmosphere. The chemical composition of the air
is the same as at sea level. Suppose you want to breathe the same partial pressure of
oxygen that you normally would breathe at sea level – no more and no less. You decide
to breathe bottled air from a tank, and adjust the composition of the air in the tank so
you get the same PO2 that you’d normally get at sea level.
2. What should the composition of the air in your tank be?
A. 21% O2, the same as at sea level.
B. 10.5% O2.
C. 42% O2.
D. It depends on the pressure in the tank. The higher the pressure, the lower
the O2 concentration should be.
Partial Pressures & dissolved gases
Suppose you open a bottle of Pepsi. Sodas like Pepsi have bubbles because they contain
a lot of CO2; they are packed with pressurized CO2.
3. What happens to the air in the top of the bottle at the moment when you
open it?
A. The PCO2 of the air in the top of the bottle suddenly increases.
B. The PCO2 of the air in the top of the bottle suddenly decreases.
4. What happens to the partial pressure gradient of CO2 between the air in
the bottle and the Pepsi at the moment when you open the bottle?
A. The partial pressure gradient increases suddenly, causing bubbles to form.
B. The partial pressure gradient decreases suddenly, causing bubbles to form.
5. What happens to the PCO2
of the soda itself after the bottle has been open
for a couple of hours?
A. The PCO2 of the soda gradually increases as bubbles form.
B. The PCO2 of the soda gradually decreases as bubbles form.

The answer I have is:
2. C
3. B
4. A
5. B