Which gases are carried by red blood cells? check all that apply. oxygen helium carbon dioxide nitrogen hydrogen

Oxygen and carbon dioxide are the gases transported by the red blood cells. Oxygen is collected from the air in the alveoli to the blood capillaries. It binds to the heme group in the haemoglobin, which confers the red colour to the red blood cells where it is contained. Carbon dioxide moves the other way around, from capillaries to alveoli. They are transported by the red blood cells when oxygen is exchanged by carbon dioxide from the cells' metabolism. 

Is that a negative 22 or a positive?