3(4)The amount of a defendant’s liability assessed under paragraph (3)(
b) may be reduced, or the proportions of liability assessed under paragraph (3)(
b) readjusted among the defendants, to the extent that a defendant proves, on a balance of probabilities, that the breach referred to in paragraph (1)(
a) did not cause or contribute to the exposure referred to in paragraph (2)(
a) or to the disease or risk of disease referred to in paragraph (2)(
b).