A cylindrical piece of wood has cross-sectional area $A$ and weight $W$. It is fully submerged in water with its axis vertical. The block is acted on by pressures $p_t$ and $p_b$ at its upper and lower surfaces respectively. What is the upthrust on the block?
- A$(p_b - p_t)$
- B$(p_b - p_t)A$
- C$(p_b - p_t)A - W$
- D$(p_b - p_t)A + W$