The correct answer is A. Particle size is decreased.
The settling velocity of a particle in a sedimentation tank is the velocity at which the particle settles under the influence of gravity. The settling velocity is proportional to the square of the particle diameter, so a decrease in particle size will result in a larger settling velocity.
The surface area of the tank does not affect the settling velocity of the particles, as the particles are settling in a fluid and not in contact with the tank walls. The depth of the tank also does not affect the settling velocity of the particles, as the particles are settling in a fluid and not in contact with the tank bottom.
Here is a more detailed explanation of each option:
- Option A: Particle size is decreased. This is the correct answer, as the settling velocity of a particle is proportional to the square of the particle diameter. A decrease in particle size will result in a larger settling velocity.
- Option B: The surface area of tank is increased. This is not the correct answer, as the surface area of the tank does not affect the settling velocity of the particles. The particles are settling in a fluid and not in contact with the tank walls.
- Option C: The depth of tank is decreased. This is not the correct answer, as the depth of the tank does not affect the settling velocity of the particles. The particles are settling in a fluid and not in contact with the tank bottom.
- Option D: None of the above. This is not the correct answer, as option A is the correct answer.