Singular or plural nouns 

Irregular Nouns That End in -Us

Some irregular nouns end in -us, like alumnus and
cactus. To make these words plural, drop the -us and add an
-i. For instance, “Many colleges request donations from
alumni.” There are, of course, exceptions to this rule. For instance, the aforementioned alumni attended different college
campuses, not college campi.

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