Which of the following is a true statement? a. electromagnetic waves consist of only changing electric fields. b. electromagnetic waves are longitudinal waves. c. electromagnetic waves do not always need a medium to travel. d. electromagnetic waves can vary in speed, wavelength and frequency.

the answer would be a

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.