Final answer:
In Python, valid variable names must begin with a letter or an underscore, and can only contain letters, numbers, and underscores. '99bottles' and 'r&d' are illegal due to starting with a number and containing a special character, respectively.
Step-by-step explanation:
In Python, variable names must adhere to certain rules to be considered valid. Here's a breakdown of the provided variable names:
- x - Valid: This is a simple, one-letter variable that is permissible.
- 99bottles - Illegal: Variable names cannot begin with a number.
- july2009 - Valid: This name combines letters and numbers, which is allowed, provided it does not start with a number.
- theSalesFigureForFiscalYear - Valid: This uses camelCase, which is a common naming convention in programming.
- r&d - Illegal: The ampersand (&) is a special character and cannot be used in variable names.
- grade_report - Valid: Underscores are allowed and are often used to separate words in variable names.
To summarize, variables in Python must start with a letter or underscore, can contain letters, numbers, and underscores, but cannot include special characters or start with a number.