Some people talk about immigration in term

问题：

Some people talk about immigration in terms of politics, some in terms of history. But the core of the matter is numbers. The Labor Department says that immigrants make up about 15 percent of the work force. It’s estimated that a third of those are undocumented workers. The Pew Hispanic Center estimates that one in four farmhands in the United States is an undocumented immigrant, and that they make up a significant portion of the people who build our houses, clean our office buildings and prepare our food.

America has become a nation dependent on the presence of newcomers, both those with green cards and those without. Business leaders say agriculture, construction, meatpacking and other industries would collapse without them.

Sure, it would be great if everyone were here legally, if the immigration service weren’t such a disaster that getting a green card is a life’s work. It would be great if other nations had economies robust enough to support their citizens so leaving home wasn’t the only answer. But at a certain point public policy means dealing not only with how things ought to be but with how they are. Here’s how they are: these people work the jobs we don’t want, sometimes two and three jobs at a time. They do it on the cheap, which is tough, so that their children won’t have to, which is good. They use services like hospitals and schools, which is a drain on public coffers, and they pay taxes, which contribute to them.

Immigration is never about today, always about tomorrow, an exercise in that thing some native-born Americans seem to have lost the knack for: deferred gratification. It’s the educated man who arrived in the Washington D. C., area and took a job doing landscaping, then found work as a painter, then was hired to fix up an entire apartment complex by someone who liked his work ethic. He started his own business and wound up employing others. Does it matter that he arrived in this country with no work visa if he is now supporting the nation’s economy

If any towns, whose aging populations were on the wane before the immigrants arrived, succeed in driving newcomers away, those who remain will find themselves surrounded by empty storefronts, deserted restaurants and houses that will not sell. It’s the civic equivalent of starving to death because you don’t care for the food. But at least everyone involved can tell themselves their town wasted away while they were speaking English.

In the author’s opinion, why do undocumented immigrants straggle for life（）

A. provide services to US citizens

B. do those the US citizens don’t want to do

C. enable their own country be robust enough to support themselves

D. make their children to be legal citizens of the US