Rules for variable names include all, but $\_\_\_\_\_\_\_\_\_\_\_\_\_.$

Group of answer choices

A variable name must start with a letter and can be a combination of letters, digits, period$(.)$ and underscore$(\_).$ If it starts with period$(.),$ it cannot be followed by a digit.

A variable name cannot start with a number or underscore $(\_)$

Variable names are case$-$sensitive $($age$,$ Age and AGE are three different variables$)$

None of the above.