Reviewing the one-to-many join

You've done some great work on one-to-many joins with this data. While pausing to reflect, you overhear a conversation about joins coming from the next cubicle. Here are four statements you hear.

• A. A one-to-many relationship exists when a column in one table matches many columns in another.
• B. The .join() method works well when joining data frames by index.
• C. One-to-many joins work best when both frames have unique values in one or more key columns.
• D. The .merge() method can join a data frame with a list of data frames.

Which of these statements are TRUE?