Why is it advised to tie any luggage kept on the roof of a bus with a rope?
A. While the bus is moving, luggage tends to remain in inertia of motion state.
B. When the bus stops, the luggage tends to resist the change and due to inertia of motion it moves forward and may fall.
C. Both a and b are true.
D. None of them is true.

Hint: In order to answer this question, we will use the concept of inertia and its different types. The property of a body by virtue of which it cannot change, by itself, its state of rest or of uniform motion in a straight line is called inertia. The different types of inertia are: inertia of rest, inertia of motion and inertia of direction.

Complete step-by-step answer:
Inertia means the resistance to change.
The different types of inertia:
1. Inertia of rest: The tendency of a body to remain in the position of rest is called inertia of rest.
2. Inertia of motion: The tendency of a body to remain in its state of uniform motion in a straight line is called inertia of motion.
3. Inertia of direction: The inability of a body to change by itself its direction of motion is called inertia of direction.
It is advised to tie the luggage kept on the roof of a bus with a rope because while the bus is in rest state, the luggage will also be in rest state. When the bus starts moving, the luggage tends to remain in its rest state and resist to change the inertia of rest. This will result in the fall of the luggage backward.
While the bus is moving, the luggage will also be in inertia of motion and when the bus stops, the luggage will tend to remain in motion and resist the change of inertia of motion. This will result in the luggage will move forward and may fall down.
Hence, we can say that While the bus is moving, luggage tends to remain in inertia of motion state and when the bus stops, the luggage tends to resist the change and due to inertia of motion it moves forward and may fall.
Therefore, the correct answer is option (C).

Note: Whenever we ask such types of questions, we should have some basic knowledge of inertia. When the bus moves, the luggage continues to remain at rest due to inertia of rest. That is why the luggage falls backward when the bus starts. As the bus stops, the luggage continues to remain in motion due to inertia of motion and falls forward.