根据以下材料，回答21-25题
When the world was a simpler place, the rich were fat, the poor were thin, and right-thinking people worried about how to feed the hungry.Now, in much of the world, the rich are thin, the poor are fat, and right-thinking people are worrying about obesity.Evolution is mostly to blame.It has designed mankind to cope with deprivation, not plenty.People are perfectly tuned to store energy in good years to see them through lean ones.But when bad times never come, they are stuck with that energy, stored around their expanding bellies.
Will public-health warnings, combined with media pressure,persuade people to get thinner,just as they finally put them off tobacco? Possibly.In the rich world, sales of healthier foods are booming and new figures suggest that over the past year Americans got very slightly thinner for the first time in recorded history.But even if Americans are losing a few ounces, it will be many years before the country solves the health problems caused by half a century's dining to excess.And,everywhere else in the world, people are still piling on the pounds.That's why there is now a consensus among doctors that governments should do something to stop them.
A valid argument for intervention is that thin people subsidize fat people through health care.If everybody is forced to carry the weight of the seriously fat, then everybody has an interest in seeing them slim down.
That should not be a problem in insurance-financed health-care systems, such as America's.Insurance companies should be able to charge fat people more, because they cost more.But group health insurance schemes, which cover most Americans,are forbidden, by law, to discriminate against fat people.The health secretary, Tommy Thompson, is trying to wiggle his way around this prohibition to allow health companies to give discounts to people on fitness programs.He should not have to：rules that prevent insurance companies charging fat people what they really cost should go.
That leaves the question of what should happen in a state-financed health system.Why not tax fattening food—sweets, snacks and take-away？That might discourage consumption of unhealthy food and recoup some of the costs of obesity.
It might；but it would also constitute too great an intrusion on liberty for the gain in equity and efficiency it represents.Society has a legitimate interest in fat, because fat and thin people both pay for it.But it also has a legitimate interest in not having the government stick its nose too far into the private sphere.If people want to eat their way to grossness and an early grave, let them.
What justifies the official involvement in obesity according to the author?

A Current health care system cannot afford fat people's costs.

B Obesity problem is not being valued by the public seriously.

C Fat people are consuming more health insurance resources.

D Individuals fail to recognize their interests in keeping fit.