What is simple harmonic motion
give two examples

S.H.M:- If the acceleration of the vibrating body directly varies with the displacement of the body from the mean position and always directed to the mean position, the motion of that body is called simple harmonic motion.

Ex:- (i) The motion of a pendulum is an S.H.M.

(ii) The motion of vibrating mass attached to a spring is an S.H.M.

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.