Which of the following forces is responsible for the flow of water in rivers?
A) Force of friction
B) Gravitational force
C) Electrostatic force
D) Magnetic force

Hint: We are asked to find the force or forces responsible for the flow of rivers. We will discuss the possibility of each given option. In other words, we will discuss the admissible reasons for an option to be true for the given situation.

Complete answer:
Here, The first option is force of friction. There is a possibility as no doubt that the force of friction itself will depend on the flow of the river but it is equivalently responsible for retaining the direction and rate of the flow of the river. Thus, we could consider this as a correct option.
Then, The option is gravitational force. This is the force which is fundamentally responsible for the flow of the river as this is the force making it hard for the river water to retain its stationary state and making it flow. Thus, this is also a correct option.
Now, Electrostatic force is nowhere responsible for the flow of the river. So this option cannot be correct.
Finally, Magnetic force also is not at all responsible for the flow of the river. So this is also an incorrect option.
Hence, The forces responsible are force of friction and gravitational force.

Thus, the correct options are (A) and (B).

Note: We considered the force of friction responsible as for the force of friction only the river sustains its motion. A student should not think that friction only opposes the motion. But friction supports the motion over opposing it.