Effects on U.S. an it's citizens (P) / Provides work force
 
Source:
"Immigration's Economic Impact." Immigration's Economic Impact. 20 Jun. 2007: 1-8. SIRS Government Reporter. Web. 22 Jan. 2015.  
Quote:

" Immigrants are a critical part of the U.S. workforce and contribute to productivity growth and technological advancement. They make up 15% of all workers and even larger shares of certain occupations such as construction, food services and health care. Approximately 40% of Ph.D. scientists working in the United States were born abroad" (Immigration's Economic Impact).  

Paraphrase:

The U.S. depends on migrants because they are critical to fulfilling the mass amounts of...

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