Answer:
Mass and volume.
Explanation:
The equation for density is always mass divided by volume. To determine the density of a fluid, you would need to find its volume and its mass.
Answer:
D. 4000 km
Explanation:
f = Frequency of wave that is being transmitted = 76 Hz
= Wavelength of wave that is being transmitted
v = The velocity of electromagnetic waves through air = 
Velocity of a wave is given by

Hence, the approximate wavelength of the waves is 4000 km
It’s c.) I think so at least
Answer: Because new theories can come out that better explain observations and experimental results can replace old theories.
Explanation: Theories more than ten years old are usually out of date. Scientists want to prove that the work of other scientists is wrong. New evidence that supports a change prompts scientists to modify earlier theories.