Is it a question or ... no ?
The answer is that they have traits that enabled them to survive in a changing environment.
During the evolution of the Earth, and later during the evolution of life, the environment was not constant. Change in the environment could lead to an extinction of some species. But, some species were able to adapt or could tolerate a wide range of environmental factors. One of the explanation, why unicellular organisms did not evolve into complex multicellular organisms is because they have traits that help them to tolerate the wide range of factors, and thus, survive in changing environment.
The correct answer is A.
Animal cells do not have a cell wall.
A cell wall is the rigid, outermost covering of plant cells and is made up of cellulose. It is absent in animal cells. The cell wall is visible under a light microscope.
Animal cells are instead covered by a cell membrane. It is made up of lipids, proteins, and small amounts carbohydrates. It is a thin and delicate structure that can only be seen using an electron microscope.
