Hi Larry,

The expression evaluates to Unknown.  It is neither true nor false.

Try this

If NULL = NULL    Print 'True'

If NOT (NULL = NULL)  Print 'False'

The problem with the question is that it asks what the WHERE clause evaluates to, which is only true if the expression is true. 

Curiously, the explanation correctly indicates that the expression evaluates to Unknown.  It's almost as if the explanation to a different question is given.