Researcher: Any country can determine which type of public school system will work best for it by investigating the public school systems of other countries. Nationwide tests could be given in each country and other countries could adopt the system of the country that has the best scores on these tests. Which one of the following is an assumption required by the researcher's argument? ['If two countries performed differently on these nationwide tests, further testing could determine what features of the school systems account for the differences.', 'A type of school system that works well in one country will work well in any other country.', 'Most countries in the research sample already administer nationwide tests to their public school students.', 'A number of children in each country in the research sample are educated in private schools.']