Select the correct answer from each drop-down menu. The human body has many structural and functional units that organize hierarchically.

Choose the correct alternatives that describe the lowest hierarchy in the level of organization.

are the basic units of matter, consisting of smaller particles (protons, neutrons, and electrons).
and neutrons reside in the nucleus, which is the central part of the atom.
carry a positive charge, and neutrons are
.
carry a negative charge, and they orbit around the nucleus.