Answer:
1. it's an atom
Explanation:
An atom is the smallest particle of an element, having the same chemical properties as the bulk element. The first accurate theory explaining the nature of matter was Dalton's Atomic Theory: 1. All matter is composed of atoms, and atoms are indivisible and indestructible.
Answer: Possession of Valves
Explanation: Veins possess valves which ensures that blood flows in only one direction. Valves helps blood to flow back to the heart against gravitational force.
Arteries don't have valves because the pressure originating from the heart is capable of causing the blood to flow in one direction.
Answer: Osteoclasts are the bone cells that break down bone.
Explanation: