Johanna is an accountant at a consumer goods company and she$$s working on merging $2$ tables. One is the products table where the columns are product$\_$id$,$ product$\_$name, serial$\_$number, date$\_$of$\_$purchase, and supplier$\_$id$.$

The second table is the suppliers table where the columns are supplier$\_$id $,$ supplier$\_$name, supplier$\_$address, and supplier$\_$phone.

What would be the secondary key in the suppliers table to link to the products table?

a$.$

supplier$\_$name

b$.$

product$\_$id

c$.$

product$\_$name

d$.$

supplier$\_$id