The answer you're looking for is A.
1. Electrons travel around the nucleus in fixed energy levels with energies that vary from level to level
2.protons, neutrons, and electrons have a mass, but electrons have far less mass
3.cations and protons
4. idk
Answer: consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder).