Which of the Following Is a Comma Pitfall

Multiple Choice

Which of the following is a comma pitfall?

A) Using commas to separate thousands, hundred thousands, millions, and so on, in numbers with more than four digits
B) Using commas to set off introductory words, phrases, and clauses
C) Using commas to set off parenthetical elements within sentences
D) Using commas to separate a subject from its predicate