The cell membrane: which of the following statements is correct?

The correct answer is B. The phospholipid bilayer of the cell membrane is selectively permeable, which means that it allows some substances to pass through it while blocking others. Small, uncharged molecules, such as oxygen and carbon dioxide, can diffuse through the cell membrane because they are small enough to pass through the phospholipid bilayer. Larger molecules, or molecules that are charged, are typically too large or charged to pass through the phospholipid bilayer and must be transported across the membrane by specialized proteins. Answer A is incorrect because not all substances can diffuse through the cell membrane. Answers B and D are incorrect because only small, uncharged molecules can diffuse through the cell membrane.