A helium balloon is attached to a top-pan balance. A metal block with a mass of $100\ \text{g}$ is put on the balance. The balance shows $91\ \text{g}$. Which conclusion may be drawn from this experiment?
- AThe balloon exerts a downward force of $0.09\ \text{N}$ on the top-pan balance.
- BThe helium has a mass of $-9\ \text{g}$.
- CThe helium has a mass of $+9\ \text{g}$.
- DThe resultant downward force on the top-pan balance is $0.91\ \text{N}$.