A certain flexible weather balloon contains helium gas. Initially the balloon is at sea level, where the temperature is 25 °C and the barometric pressure is 720 torr. The balloon then rises to an altitude of 6000 ft, when the pressure is 605 torr and the temperature is 15 °C. What is the change in volume of the balloon as it ascends from sea level to 6000 ft?
a.15 V1 = V2
b. 0.75 V1 = V2
c. 1.21 V1 = V2
d. 0.71 V1 = V2