Answer: 0
Explanation: The set of counting numbers, {1, 2, 3, 4, ...}
is called the set of natural numbers.
When we include 0 in the set of natural numbers,
the new set, {0, 1, 2, 3, ...} is called the set of whole numbers.
Notice that all natural numbers are
included within the set of whole numbers.
We can represent the relationship between these two
sets of numbers using the diagram shown below.
In fact, 0 is the only whole number that's not a natural number.