In an experiment, two-year-old boys and their fathers
made pie dough together using rolling pins and other
utensils. Each father-son pair used a rolling pin that
was distinctively different from those used by the
other father-son pairs, and each father repeated the
phrase "rolling pin" each time his son used it. But
when the children were asked to identify all of the
rolling pins among a group of kitchen utensils that
included several rolling pins, each child picked only
the one that he had used.
Which one of the following inferences is most
supported by the information above?
(A) The children did not grasp the function of a
rolling pin.
(B) No two children understood the name "rolling
pin" to apply to the same object.
(C) The children understood that all rolling pins
have the same general shape.
(D) Each child was able to identify correctly only
the utensils that he had used.
(E) The children were not able to distinguish the
rolling pins they used from other rolling pins.

Can you please explain me the expression: No two children understood?
Many thanks in advance