Why does the NIST guide advise writing large numbers as powers of ten instead of using words such as 'billion'?
Because 'billion' may only be used in scientific papers
Because 'billion' is an SI prefix that has been withdrawn
Because the SI forbids number words in any context
Because the names of large numbers differ from country to country✓Correct answer
Explanation
In many countries a billion means 10¹², while in the United States it means 10⁹, so the word is ambiguous to an international reader. Writing 10⁹ removes the doubt. 'Billion' is a number word, not an SI prefix at all.