Statement-1: Two particles moving in the same direction do not lose all their energy in a completely inelastic collision. Statement-2: Principle of conservation of momentum holds true for all kinds of collisions.

Statement 1 is true. In a completely inelastic collision, the two particles stick together and move with a common velocity. While kinetic energy is not conserved, some kinetic energy remains after the collision if the particles are moving in the same direction initially. The kinetic energy lost is converted into other forms of energy, such as heat or sound.

Statement 2 is also true. The principle of conservation of momentum states that the total momentum of an isolated system remains constant if no external forces act on it. This principle holds true regardless of the type of collision (elastic or inelastic).

However, Statement 2 is not the correct explanation for Statement 1. The fact that momentum is conserved doesn't directly explain why some kinetic energy remains in a completely inelastic collision of particles moving in the same direction. The reason lies in the vector nature of momentum and velocity. While kinetic energy is a scalar and only depends on the magnitude of velocity, momentum depends on both magnitude and direction of velocity. In a completely inelastic collision, momentum is conserved, leading to a final velocity that results in some remaining kinetic energy if the initial velocities are in the same direction. Therefore, the correct answer is Statement-1 is true, Statement-2 is true; Statement-2 is not the correct explanation of Statement-1.