An air bubble is moving upward through a tank of water at terminal (constant) velocity. The forces acting on the bubble are $X$, $Y$ and $Z$, as shown. The upthrust on the bubble is $X$. Which statement about the forces is correct?
- A$Z$ is the viscous force on the bubble, $Y$ is the weight of the bubble and $X = Y + Z$.
- B$Z$ is the viscous force on the bubble, $Y$ is the weight of the bubble and $X > Y + Z$.
- C$Z$ is the weight of the bubble, $Y$ is the viscous force on the bubble and $X = Y + Z$.
- D$Z$ is the weight of the bubble, $Y$ is the viscous force on the bubble and $X > Y + Z$.