Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
E XA MP L E 1 1. The most common carbon atoms have six protons and six neutrons in their nuclei. What are the atomic number and the mass number of these carbon atoms?.


Question: The most common carbon atoms have six protons and six neutrons in their what?
nuclei
nuclei.