Simple animals are able to supply their cells with nutrients through diffusion and active transport across cell membranes because a. they have complex circulatory systems.
b. their cells are in direct contact with the environment.
c. their cells are not in direct contact with the environment.
d. their blood transports oxygen and other materials through a series of blood vessels.

The correct answer is option B) their cells are in direct contact with the environment.

By simple animals we mean unicellular organisms or organisms which have very low grade of organisation. Such organisms do not have three germ layers they either have 2 germ layers or do not have germ layers at all that means that they are comprised of only one cell. Since, they do not have many germ layers in their body their cells are in direct contact with the environment. Their cells can directly obtain nutrients form the environment through the process of diffusion. But, in complex organisms the cells do not obtain nutrients directly from the environment. They have specialized organs and organ systems to accomplish a particular task which increases their efficiency. They have well organized circulatory system which supplies nutrients to the cells/tissues of the body.