The country"s biggest challenge now is the plight of lower-income Americans, who are under severe and sustained economic pressure. Today, America resembles a tale of two cities. Those who own homes or stocks have benefited from the recovery in these asset classes and are moving up again. But 40% of working-age families earn $ 40,000 a year or less. Generally they live within 250% of the official poverty level, which is the eligibility threshold for food stamps. Indeed, judging from current trends, half of today"s 20-year-olds will receive food stamps during their lives. More broadly, median household income is still 8% below the precr level, and those who have not completed college are seeing declines in anticipated lifetime earnings compared with their peers with college degrees. This is the primary economic challenge. If a third of the population has little purchasing power, it will be hard to achieve the desired rate of long-term growth. The US needs to improve the work skills of this group, strengthen the social safety net and increase the number of young Americans receiving a full college education. Although doing more to relieve the financial burdens of working Americans is good economics, it is also, and perhaps more important, a matter of values. For much of the 20th century Americans strove, with much success, to build a fairer and more inclusive society. But today, too many working families are living paycheck to paycheck or even in outright poverty, while the toeholds(克服困难的办法)to economic stability become fewer and farther between. With the economy"s near and medium term economic outlook strong, now is the time to remove the barriers that are keeping hardworking Americans walking a far too thin financial line. The biggest challenge that America faces is______.