A helium balloon has a flexible skin of negligible weight and infinite capacity for expansion, so that the helium is always at the same pressure as the surrounding air. If the balloon moves up and down slowly, then the temperature of the gas in the balloon will be practically the same as that of the surrounding air. If the mass of helium in the balloon is 10 lbm, how much payload can it lift under the following conditions:

(a) 1 atm and 70°F,

(b) 0.01 atm and 0°F, and

(c) 0.001 atm and −100°F? Assume that helium behaves as an ideal gas.