L claims that her idea could save 5000 lives a year. T finds this implausible as it would mean that the population in that counterfactual would be 50000 higher in ten years. This is a weak argument, as there is no reason that a population that is a mere 50k larger is impossible to fathom. It is also weak since a good num bar of those 50k lives saved would have died of other causes in those 10 years.
The question asks us how L would best respond.
B undermines the idea that t finds implausible. Saving 5000 per year does not actually mean that the population would be 50,000 higher after ten years, because a good chunk of them would have dies of other causes during that period. Thus, the implication that T finds implausible is actually not as large as she thought.