The answer is 20. Duplicated chromosome has two sister chromatids, identical copies of the chromosome. These two chromatids are joined at the region called centromere. Therefore, one duplicated chromosome has one centromere and two sister chromatids. If there are 20 duplicated chromosomes in a cell, there will be 20 centromeres.
The biggest airlines flying to United States: American Airlines flights to United States, Delta flights to United States, Southwest Airlines flights to United States. Regions in the USA - United States. Airlines flying to the USA - United States - by city.
As of July 2016, American Airlines operates a fleet of 952 aircraft, making it the largest commercial fleet in the world. It operates a mix of Airbus, Boeing, Embraer, and McDonnell Douglas aircraft. Over two thirds of American's aircraft are narrow-bodies, mainly Airbus A320 series and Boeing 737-800.