Identify invalid variable names from the following, giving reason for each:

Group, if, total marks, S.I., volume, tot_strength, #tag, tag$\text{\textdollar}$, 9a, for

1. Group — This is a valid variable name.

2. if — This is an invalid variable name because if is a reserved keyword in Python and cannot be used as a variable name.

3. total marks — This is an invalid variable name because it contains a space, which is not allowed in variable names. A variable name must be a single continuous string without spaces.

4. S.I. — This is an invalid variable name because it contains full stops, which are not allowed in variable names. Variable names can only include letters, digits, and underscores.

5. volume — This is a valid variable name.

6. tot_strength — This is a valid variable name.

7. #tag — This is an invalid variable name because it starts with a hash symbol (#), which is not allowed in variable names. Variable names must start with a letter or underscore.

8. tag$\text{\textdollar}$ — This is an invalid variable name because it contains a dollar sign ($\text{\textdollar}$), which is not allowed in variable names. Variable names can only include letters, digits, and underscores.

9. 9a — This is an invalid variable name because it starts with a digit, which is not allowed in variable names. Variable names must start with a letter or underscore.

10. for — This is an invalid variable name because for is a reserved keyword in Python and cannot be used as a variable name.