The "sea of instability" refers to a region of elements on the periodic table that are highly unstable. These elements have extremely short half-lives that may be measured in micro- or nanoseconds. (A nanosecond is the time it takes for light to travel one foot.) This region of unstable elements surrounds the island of stability.
3rd one sorry if it’s wrong
Sorry this will probably be pretty long.
So think of the "control" as being something you yourself add to increase or decrease the effects in an experiment.
I'll give you an example so it is not as confusing.
Say you have decided to make an experiment on plants. Which plant can grow the fastest on which type of liquid? What is being added to this experiment? The liquid! Or all of the liquids you used. Like if you used Coke, Lime Gatorade, Orange Gatorade, and Water. Each drink will EFFECT each plant differently.
Hope I was of any hope?