Answer: C. They both thought that atoms could not be broken down into smaller particles
Step-by-step explanation:
Democritus and Dalton both believed that atoms are the tiny invisible particles. These are the smallest particles in nature. These exists permanently in nature without changing. These atoms are so tiny that they cannot be broken down into sub-particles. The atoms made up of different materials are said to be different in nature. Hence, C is the correct option.