Summarization
Directions: The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Spell out the missing letters of the word. 

In the U.S. , cases of polio are now extremely rare, and ironically, are almost always caused by the Sabin vaccine itself——being live, the virus can mutate to a stronger form. Elsewhere there are still about 250,000 cases per year, mostly in developing nations where vaccination has not become widespread. The World Health Organization has goals to eradicate polio completely in the first decade of the twenty-first century.
The d of polio.