A tricky QC problem

In a town of 100 households, every family has a son, a daughter, both or none.
80 households have a son and 40 households have a daughter.
Quantity A: The number of households with both a son and a daughter.
Quantity B: 20
The suggested answer (from tickbox quiz) is D.

But wouldn’t it be A?
Answer’s going to be always greater than 20 if we consider the number of families with no son and daughter as non-zero (starting from at least 1). From the language of the question is it possible to consider that number as 0 (zero)?

Yeah