7.
What is meant by independent
Independent behaviour of waves?

Its AnsweredHere it is

A wave  preserve its individuality while travelling through space.So when a Number of waves travel through a region at same time, each wave travels independently of the others i.e., as if all other waves were absent. That is why, with so many different musical instrument playing simultaneously in a full orchestra , we can still identify the note produced by an individual instrument. An important consequence of the independent behaviour of the Waves is the principle of superposition of waves.

Explanation:

answer;

d. are made mostly of liquid

explanation;

-the general   composition and structure planets includes; they have a thick gas and liquid layer covering a small,solid core. mostly the gases are hydrogen and helium atmosphere and layers of liquid hydrogen. for instance neptune and uranus are examples of the outer planets, they have a core covered by ice and ammonia.

-neptune's atmosphere is hydrogen and helium with a bit of methane so there is no oxygen to breath and the interior is made up of frozen water and ammonia with a rock and iron core. thus, there is no liquid water or solid surface to survive.