Final answer:
C) Millions
Millions of atoms can fit across the period at the end of a sentence since it takes about fifty million atoms to make a line that is 1 cm long.
Step-by-step explanation:
The question asks how many atoms fit across the period at the end of a sentence. Considering that individual atoms are extremely small, with about fifty million of them needed to make a line that is just 1 cm long, one can deduce that a period, being much smaller than 1 cm, contains several million atoms, as mentioned in the referenced material. Therefore, the correct answer to the question would be C) Millions.