Final answer:
The mass of oil spilled into a lake will remain constant initially, but can decrease over time due to evaporation of the lightest hydrocarbons and decomposition by bacteria, resulting in a smaller amount of oil remaining on the surface of the water.
Step-by-step explanation:
When a boat engine leaks oil into a lake, the mass of the spilled oil is initially constant. however, certain processes can change this over time. Crude oil, which is less dense than water and does not mix with it, will float to the water's surface when spilled. Over time, the lightest hydrocarbons in the oil may evaporate, which pollutes the air but reduces the mass of the oil remaining on the water's surface. Moreover, bacteria can decompose the oil further, a process that can take many years, and may result in only about 15% of the original volume remaining, often in the form of thick asphalt lumps that can be particularly harmful to wildlife.
Thus, the correct answer to the question is: b. The mass of the spilled oil will remain constant initially but may decrease over time due to evaporation and biodegradation processes.