A balloon is launched at sea level, where the air pressure is $100 \mathrm{kPa}$. The density in the hot-air chamber is $1.0 \mathrm{~kg} / \mathrm{m}^{3}$. What is the density of the air when the balloon has risen to a height where the atmospheric pressure is $33 \mathrm{kPa} ?$
A. $3.0 \mathrm{~kg} / \mathrm{m}^{3}$
B. $1.0 \mathrm{~kg} / \mathrm{m}^{3}$
C. $0.66 \mathrm{~kg} / \mathrm{m}^{3}$
D. $0.33 \mathrm{~kg} / \mathrm{m}^{3}$