Complete the wedding planner's frantic text message.
The _________________________ are arguing about the floral arrangements again, and two _________________________ just wandered in from the street and tried to steal a slice of the wedding cake!
The correct answers are mothers-in-law and passersby.
For hyphenated compound nouns containing a noun and a preposition/adjective (like mother-in-law or passerby), we make them plural by adding "-s" to the principal noun (the "head noun"), not to the end of the word.
Therefore, mother becomes mothers (mothers-in-law) and passer becomes passers (passersby).