d. an opening specified for waste removal is present
B) The positive charge of the protons in the nucleus equals the negative charge in the electron cloud.
(I just took the test and it was right). :)
Answer:
Cells are considered the basic units of life in part because they come in discrete and easily recognizable packages. That's because all cells are surrounded by a structure called the cell membrane — which, much like the walls of a house, serves as a clear boundary between the cell's internal and external environments.