When the balloon reaches the highest point of its flight (around 2000 m), the air pressure has dropped to 80% of the pressure at ground level, and the density has dropped to 85% of the previous value. What temperature, in degrees Celsius, must the air in the balloon be to keep it floating at this altitude?
a) 0°C
b) -10°C
c) -20°C
d) -30°C
