Rules for plural formation:

• • Nouns ending in s, x, z, ch or sh are made plural by adding -es. E.g. witch, dish, box, class.
• • Nouns ending in -y are made plural by replacing -y with -ies; E.g. candy- candies, city- cities, fairy- fairies.
• • Some nouns that end in -f or -fe become plural by changing -f or -fe to -ves; E.g. wife- wives, leaf-leaves.
• • Some nouns that end in -f or -fe can be made plural by adding -s; E.g., chief- chiefs, proof- proofs.
• • Some nouns ending in -is can be made plural by changing -is to -es; E.g. crisis- crises, thesis- theses.

One noun in each group does not follow the same rule as the others for plural formation. Find it.

Example: casino / ratio / hero / promo