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

1. Group — It is a valid variable name.
2. if — It is a reserved keyword in Python and cannot be used as a variable name.
3. total marks — It contains a space, which is not allowed in variable names.
4. S.I. — It contains a dot (.), which is not allowed in variable names.
5. volume — It is a valid variable name.
6. tot_strength — It is a valid variable name.
7. #tag — It starts with a hash (#), which is not allowed in variable names.
8. tag$\text{\textdollar}$ — It contains a dollar sign ($\text{\textdollar}$), which is not allowed in variable names.
9. 9a — It starts with a number, which is not allowed in variable names.