two P vs two P's : correct use , example

Two P vs  Two P's  :  which one is correct

RULE- numbers (संख्याएं.1, 2,3,4------ ),letters( अक्षर), तथा Word (शब्द )का plural apostrophe(‘s) लगाकर बनाई जाती है |              

Example : There are two P in the word apple.

Correct Sent-  There are two P's in the word apple.

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