Final answer:
The ship displaces 1200 m³ of seawater, which is equal to its mass of 1200 tonnes. If it enters freshwater, to maintain the same displacement volume, it must unload 30 tonnes of cargo because of the density difference between seawater and freshwater.
Step-by-step explanation:
The question is about calculating the volume of seawater displaced by a ship and adjusting cargo mass when moving from seawater to freshwater to maintain the same level of displacement. According to Archimedes' principle, the volume of water displaced by an object is equal to the volume of the object that is submerged. In this case, for the ship to float, it must displace a volume of water equal to its own mass. Since the mass of the ship is 1200 tonnes and knowing that seawater has a density of approximately 1 tonne per cubic meter, the volume of seawater displaced is 1200 m³. When the ship enters freshwater, which has slightly lower density, the ship will displace a greater volume of water if it's loaded with the same mass. To maintain the same displacement volume as before, which is 1200 m³, the ship must unload some cargo to compensate for the density difference. Given that the density of freshwater is approximately 1000 kg/m³ (1 tonne/m³), and seawater is generally about 1025 kg/m³, the difference is 25 kg/m³. Therefore, to maintain the displacement volume, the ship would need to unload 1200 m³ * 25 kg/m³ = 30000 kg or 30 tonnes of cargo. The correct answer is A. 1200 m³, 30 tonnes.