Number one is the question i need on!

What is the chemical formula of the following compound

Which of the following is a compound? a. carbon b. oxygen c. hydrogen d. water

Which of the following statements is true? a. elements in the last period are radioactive. b. atomic weight is the same as atomic mass. c. elements in the same group have the same number of electron shells. d. atomic number equals the number of neutrons in the nucleus of an atom.