Given a scientific question, generate a correct answer to it.

Example Input: Despite its small size, an erythrocyte contains about 250 million molecules of what?
Example Output: hemoglobin

Example Input: What do plants make to store extra glucose?
Example Output: starches

Example Input: The radius of an atom is defined as one-half the distance between the nuclei in a molecule consisting of two identical atoms joined by what?
Example Output:
covalent bond