Which of the following is NOT a difference between a unique index and a primary key?

A.
A primary key is a special form of a unique constraint; both use a unique index.

B.
Unique indexes can be defined over one or more columns; primary keys can only be defined on
a single column.

C.
A table can have many unique indexes but only one primary key.

D.
Unique indexes can be defined over one or more columns that allow null values; primary keys
cannot contain null values.