What does the 19 mean in fluorine-19?

Atrain travels 74 kilometers in 3 hours, and then 81 kilometers in 5 hours. what is its average speed?

Which sentence about particles in matter is true? a. atoms are present in solids and liquids but not in gases. b. the particles of matter are in constant motion. c. the same kinds of atoms are found in different elements. d. when a solid changes to a liquid, the sizes of the particles change.

Answer asap : ( a. how does mucus prevent the entry of pathogens? b. describe two ways white blood cells protect us from pathogens.