A Hot Air Balloon Along with Its Cargo Has a Mass

A hot air balloon along with its cargo has a mass of $4.0 \times 10 ^ { 5 } \mathrm {~kg}$ and it holds $7.0 \times 10 ^ { 5 } \mathrm {~m} ^ { 3 }$ of hot air. It is floating at a constant height in air with a density of $1.29 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
What is the density of the hot air in the balloon?

A) $0.57 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
B) $1.4 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
C) $0.86 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
D) $0.43 \mathrm {~kg} / \mathrm { m } ^ { 3 }$
E) $0.72 \mathrm { kq } / \mathrm { m } ^ { 3 }$