The unique constraint differs from the primary key constraint in what way? â

The following question has multiple choices as follows

a. The UNIQUE constraint ensures that a specific condition is true before a data value is added to a table.

b. The UNIQUE constraint can be created at either the column level or the table level.

c. The UNIQUE constraint allows NULL values.

d. The UNIQUE constraint does not allow NULL values.

The answer is (C) The UNIQUE constraint allows NULL values.

The unique constraint differs from the primary key constraint in that it allows Null values. Primary key is not nullable while Unique constraints may be nullable. If for instance you try to make a nullable column as your primary key, SQL server will first convert it into a NOT NULL and then create the primary key.

According to my brain it's a rational number because it can be put as a fraction