Alternative energy sources could curtail the use of fossil fuels. why are they not extensively used?

They have faults. The solar panels are expensive to put in and cause taxes to go up. Wind isn't powerful enough. Technology needs to advance further for other types of power so it can lose many of their faults and become common to use as an alternative to fossil fuels.

True as the lakes are a marine body. also there's no difference in lake and ocean except for the size.

Yes that is correct answer in 6