The general rule for forming possessives
The general rule is that the possessive of a singular noun is formed by adding an apostrophe and s, whether the singular noun ends in s or not.
Ex) a week’s vacation
Texas’s oil industry
The possessive of a plural noun is formed by adding only an apostrophe when the noun ends in s, and by adding both an apostrophe and s when it ends in a letter other than s.
Ex) the twins’ parents
the alumni’s fundraisingfrom The Punctuation Guide
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