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If the denominator is 0 then the fraction is undefined.
0. Whatever the denominator - unless the denominator is also 0 in which case the fraction is undefined.
The fraction reduces to 1 - UNLESS they are both zero, in which case the fraction is undefined.
A quotient in which the numerator or denominator are undefined will be undefined. For example p/q is an undefined quotient until you know something about p and q. Also, if the denominator is zero, the division is undefined.
YES a fraction can be a denominator.