An American educational expert, Frederick M. Hess, says that a long summer vacation doesn't make sense in today's world. This summer, 50 million kids will spend much time talking online, watching TV, playing video games and asking older brothers or sisters to take them to the supermarket. They will also be putting their academic futures at risk. During much of the 20th century, many Americans worked in factories that did not need a college degree but still offered a high salary. Also, in the 19th century, there was no air condition. And poor hygiene   meant that crowded schools had health risks. Things have changed. For today's students, academic skills are key to future success. Many countries don't give kids an American-style summer vacation. They offer no more than seven consecutive   weeks of vacation. Most American school districts offer up to 13 weeks. To compete in global marketplace, Americans must be prepared to go up against international competitors. Summer vacation also causes challenges for today's families. In the 1960s, more than 60% of families had a stay-at-home mum. Now, two-thirds of American children live families where every adult works. For these families, summer vacation can be more burden than break. Someone must watch the kids. But the biggest problem may be how summer vacation hurts academic achievement. Scientist have found that disadvantaged students lose important ground in the summertime. A school year can allow time-limited teachers to conduct richer and more imaginative lessons. Schools would have more time for sports, languages, music and the arts. Before the 20th century, a long summer vacation might   _  .
reduce income for Americans reduce health risks for students cause career failure for Americans cause loss in global market for America