Quiz: Extract column from a data frame

Output
      rank discipline yrs.since.phd yrs.service  sex salary
1     Prof          B            19          18 Male 139750
2     Prof          B            20          16 Male 173200
3 AsstProf          B             4           3 Male  79750
4     Prof          B            45          39 Male 115000
5     Prof          B            40          41 Male 141500
 [ reached 'max' / getOption("max.print") -- omitted 392 rows ]
Which R command can be used to calculate the average salary of professors in the Salaries dataset from the carData package?
Extract or replace columns in a data frame using `$`