What do you need to consider when creating a secondary index on a table? There are 2 correct answers to this question.

What do you need to consider when creating a secondary index on a table? There are 2 correct answers to this question.
A . The index can be created for specific database systems only
B . The most frequently selected fields should be at the first positions in the index
C . The index must always be unique
D . The table will be updated more quickly if you create more indexes

Answer: B,C