C) light waves travel faster than sound waves
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The moment of inertia is 
Explanation:
The total moment of inertia of the system is the sum of the moment of inertia of the rod + the moment of inertia of the two balls.
The moment of inertia of the rod about its centre is given by

where
M = 24 kg is the mass of the rod
L = 0.96 m is the length of the rod
Substituting,

The moment of inertia of one ball is given by

where
m = 50 kg is the mass of the ball
is the distance of each ball from the axis of rotation
So we have

Therefore, the total moment of inertia of the system is

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According to Newton's Second Law of Motion, the net force experienced by the system is equal to the mass of the system in question times the acceleration in motion. In this case, the net force is the difference of gravitational force and the force experience by the motion of the airplane. This difference is already given to be 210 N.
Net force = ma
210 N = (73 kg)(a)
a = +2.92 m/s²
Thus, the acceleration of the airplane's motion is 2.92 m/s² to the positive direction which is upwards.
Missing question: "What is the spring's constant?"
Solution:
The object of mass m=6.89 kg exerts a force on the spring equal to its weight:

When the object is attached to the spring, the displacement of the spring with respect to its equilibrium position is

And by using Hook's law, we can find the constant of the spring:
You can find the mass of an atom by adding the number of protons and neutrons. In this case the atom has 2 protons and 3 neutrons so the mass is 5.