Look at these examples:
Note that nouns ending in -au, -eau take an -x in the plural form, and not an -s.
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Examples and resources
"Les pluriel des noms" by youlearnfrench.blogspot.com
Q&A Forum 2 questions, 3 answers
How is the question "What does ''noyau'' mean?" relevant to the lesson?
'Noyau' is one of those words ending in 'au' whose plural is a silent 'x' .
Hope this helps!