You are designing a MongoDB schema to support queries that will include lookups. What should you do?
a. Create an index on the key value used as the primary key.
b. Create an index on the key value used as the foreign key.
c. Create a multicolumn index on the key value used as the foreign key and the most unique column in the document.
d. Create a multicolumn index on the key value used as the primary and also the forign key.
