Many countries face a somewhat more serious economic problems in the form of an unfavorable trade balance (贸易逆差) with other nations. Such an imbalance exists when the total value of a country’s imports exceeds that of its exports. For example, if a country buys $250 billion of products from other countries, yet sells only $100 billion of its own products overseas, its trade deficit is $150 billion. Many underdeveloped nations find themselves in this position because they lack natural resources or the industrial capacity to use these resources, and thus have to import raw materials or manufactured goods.
One effect of a trade deficit is the flow of currency (货币) out of a country. In the case of an underdeveloped nation, this can cause many financial difficulties, including failure to meet debt payments and obstacles to creation of an industrial base. Even in the case of a fully developed nation such as the United States, a large trade deficit is reason for alarm. American products, made by well-paid workers in US industries, cost more to produce than those made in places lille Asia, where labor and material costs are much lower. Money spent on foreign products is money not spent on items produced by domestic industries.What is the passage mainly about（）

A．Several worldwide economic problems.
B．The causes and consequences of trade deficit.
C．Lack of resources in underdeveloped countries.
D．The value of exports against imports.