Suppose you have $2$ data tables. One is the students data table where the columns are student$\_$id$,$ first$\_$name, last$\_$name, date$\_$of$\_$birth, and advisor$\_$ The second table is the advisors table where the columns are advisor$\_$id$,$ advisor$\_$first$\_$name, advisor$\_$last$\_$name, and advisor$\_$phone. You want to combine the two tables.

What would be the secondary key in the advisors table to link to the student table?

a$.$ last$\_$name

b$.$ advisor$\_$id

c$.$ student$\_$id

d$.$advisor$\_$name