The diagram presents two blocks $X$ and $Y$. Block $X$ has edges each of length $a$. When block $X$ is put into a liquid of density $\rho$, with the shaded face level with the liquid surface, it experiences an upthrust $U$. Block $Y$ has horizontal sides measuring $a$ and $2a$ and a height of $\frac{1}{2}a$. Block $Y$ is placed in a liquid of density $2\rho$, again with the shaded face level with the liquid surface. What is the upthrust on block $Y$?
- A$\frac{1}{2}U$
- B$U$
- C$2U$
- D$4U$