Which of the following are true statements about red blood cells (erythrocytes)?
a. mature cells do not have a nucleus
b. new rbc’s may enter the blood stream at the rate of 2 million/second
c. the primary molecule within the cytosol is keratin
d. their plasma membrane is both strong and flexible
e. besides oxygen, they also transport carbon dioxide molecules.

The correct answer will be options A, B and E.

Explanation:

Red blood cells or RBC or erythrocytes are the cells present in the connective tissue which forms the blood. RBC perform various functions in the body but the primary function is the transport of the gases in the body.

The RBC are continuously formed in the bone marrow region of the bone form the hemopoietic stem cell found in the bone marrow. These cells produce a large amount of RBC that is about  2 million cells per second in a healthy adult.

When RBC are formed posses nucleus but when mature, they lack nucleus that is genetic material and organelles like mitochondria so, they are not able to divide.  

These RBC contain haemoglobin in their cytoplasm which shows high affinity to bind oxygen and low affinity to bind carbon dioxide to the iron group of haemoglobin.

Thus, options A, B and E are the correct answer.