In a normal human body, the total number of red blood cells is

The correct answer is B. 20 trillion.

Red blood cells are the most common type of blood cell and they are responsible for carrying oxygen to the body’s tissues. They are made in the bone marrow and they have a lifespan of about 120 days. The average adult has about 20 trillion red blood cells in their body.

Option A is incorrect because it is too low. Option C is incorrect because it is too high. Option D is incorrect because it is the same as option A.