Which of the following statements about using indexes in MySQL is true?
a) Indexes can only be created on individual columns, not a combination of columns.
b) Increasing the number of indexes in a MySQL database speeds up update operations.
c) The values in an index are maintained in sorted order to allow speedy access to the unsorted data on which the index is based.
d) It is not possible to create more than one index on the same table in a MySQL database.
