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The `lower()` method is used to convert all characters in a string to lowercase. Therefore, to convert 'Hello World!' to all lowercase, use the 'Hello World!'.lower() method.

The `strip()` function is used to remove whitespace from the start and end of a string. Therefore, to remove whitespace from both sides of '   Hello, World!   ', use the '   Hello, World!   '.strip() function.

The `isalnum()` method is used to check if a string contains only alphabetic characters and numbers. Therefore, to check if the string 'Python3' contains only alphabetic characters and numbers, use 'Python3'.isalnum() method.

The `find()` function searches for a specific character or substring from the start of the string and returns the index of its position. Therefore, use the 'find()' function to find the first occurrence of the word 'World' in 'Hello, World! Welcome to the World of Python.'

The `split()` method is used to split a string based on a specific delimiter. In this case, to split 'apple, banana, cherry' by comma and space (', '), use the 'split(', ')' method.

The `in` operator checks whether a specified substring or character is present within a string and returns True if it exists. Therefore, to check if the word 'World' exists in 'Hello, World!', use 'World' in 'Hello, World!' expression.

Please choose from the answers below.