Question on Comparision of null values in SQL

#1

In SQL, relations can contain null values, and comparisons with null values are treated as unknown. Suppose all comparisons with a null value are treated as false. Which of the
following pairs is not equivalent? (GATE CS 2000)
(a) x = 5, not (not (x = 5)
(b) x = 5, x > 4 and x < 6, where x is an integer
© x < 5, not(x = 5)
(d) None of the above

#2

how null come into picture

#3

if x = null,then
i think c is correct
null < 5 -> false
not(null = 5) not(false) true
so c,this was easy !!