D. All of these are correct.
Work Step by Step
The cell theory states that 1. A cell is the basic unit of life. A single cell exhibits all of the basic characteristics of life. 2. All living organisms are made up of cells. Some organisms are single celled, while others (like humans) are made up of multiple cells. 3. New cells arise only from preexisting cells. New cells cannot be spontaneously created from non-living objects. When humans reproduce a living sperm cell bonds with a living egg cell to form a zygote.