Assume you have a data frame that contains a string variable called 'phone'. The phone numbers in this variable appear in $($###$)$ ###$-$

#### or ###$-$###$-$#### format. Which feature of R can you use to isolate the area code $($the three numbers between the parentheses or

the first three numbers$)?$

A regular expression.

A string operation.

It is not possible to do this using $R.$

A mathematical operation.