1) 4°C : It has the highest density as shown on the graph.
2) Water expands when it freezes, making it less dense than just water.
3) The ice would sink to the bottom, then the rest of the water would freeze as well, the entire lake/river/whatever will freeze eliminating the organisms that live there.
Answer:

Explanation:
For the cat to stay in place on the merry go round without sliding the magnitude of maximum static friction must be equal to magnitude of centripetal force

Where the r is the radius of merry-go-round and v is the tangential speed
but

So we have

Substitute the given values
So

Move the objects faster to get more friction.
I calculated on a sheet and used differentiation to solve the problems.