Answer the following question given this paragraph: 

Figure 39.19 The protein inside (a) red blood cells that carries oxygen to cells and carbon dioxide to the lungs is (b) hemoglobin. Hemoglobin is made up of four symmetrical subunits and four heme groups. Iron associated with the heme binds oxygen. It is the iron in hemoglobin that gives blood its red color.


Q: What does hemoglobin in blood carry to the cells?

 Choices:

- methane

- carbon dioxide

- nitrogen

- oxygen

A:
oxygen
