<PRE><B>Question 174659</B>
d) is the answer
:

Rational numbers are closed under addition, subtraction, multiplication, as well as division by a nonzero rational.



A set of elements is closed under an operation if, when you apply the 
operation to elements of the set, you always get another element of 
the set. 

For example, the whole numbers are closed under addition, because if 
you add two whole numbers, you always get another whole number - there 
is no way to get anything else. 

But the whole numbers are _not_ closed under subtraction, because you 
can subtract two whole numbers to get something that is not a whole 
number, e.g., 

  2 - 5 = -3
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