The correct answer is D. 1 : 500.
Red blood cells (RBCs) and white blood cells (WBCs) are two types of blood cells. RBCs are responsible for carrying oxygen to the body’s tissues, while WBCs are responsible for fighting infection.
The average number of RBCs in a healthy adult is 4.5 to 5.5 million per microliter of blood. The average number of WBCs in a healthy adult is 5,000 to 10,000 per microliter of blood.
This means that the ratio of RBCs to WBCs in a healthy adult is about 1:500.
Option A is incorrect because it states that the ratio of RBCs to WBCs is 15:500. This is much higher than the actual ratio.
Option B is incorrect because it states that the ratio of RBCs to WBCs is 10:500. This is also much higher than the actual ratio.
Option C is incorrect because it states that the ratio of RBCs to WBCs is 5:500. This is closer to the actual ratio, but it is still slightly higher than the actual ratio.