In most schools of the United States, students attend classes for 5 days each week. Schools are required to hold classes for about 180 days a year. Recently, schools in some areas of the country have begun experimenting with the new teaching arrangement of holding classes for 4 days a week. Classes would meet either Monday through Thursday, or Tuesday through Friday. Some schools in 12 American states are now using that schedule（日程安排）.
The students still have the same number of total hours in class as before. But they attend school for a longer time on each of the 4 days. In Colorado, schools on the four-day week hold classes for seven-and-one-half hours a day for 144 days. Traditional（传统的）schools meet for 6 hours a day for 180 days.
Many schools on a four-day schedule offer special programs, trips or sports meets on the fifth day. Officials say this gives students even more time in class than on the traditional schedule. Students do not have to miss school to go to the events. They can also go to doctors or deal with other important affairs on the fifth day.
The main reason for shortening the school week is that many small schools can save money by not providing meals or transportation for students on the fifth day. And they say research shows that students are not harmed by shorter week.
What change does the four-day school week bring about?（ ）
根据第三段内容，可得知同学们可以参加更多的课外活动，同时也不会耽误课程，故选C。
把10gMgCl2和MgSO4的混合物溶于适量水中，滴加2mol／L的BaCl2溶液至沉淀完全，共消耗BaCl2溶液25mL。求混合物中MgCl2和MgSO4的质量各是多少克
固定在匀强磁场中的正方形导线框abcd，各边长为1．其中ab是一段电阻为R的均匀电阻丝，其余3边均为电阻可忽略的铜线。磁场的磁感应强度为B，方向垂直纸面向里。现有一与ab段的材料、粗细、长度都相同的电阻丝PQ架在导体框上，如图所示。PQ以恒定速度方向如何
一小物块以速度v0=10m／s沿光滑地面滑行，然后沿光滑曲面上升到顶部水平的高台上，并由高台上飞出，如图所示。问当高台的高度h多大时，小物块飞行的水平距离最大这个距离是多少(g=10m／s2)
某晶体盐的水溶液呈浅绿色，在此溶液中通入氯气，溶液变为棕黄色，把所得棕黄色溶液分成两份：一份加入KSCN溶液，溶液变深红色；另一份加入BaCl2溶液，产生不溶于稀HNO3的白色沉淀，则该晶体盐的化学式是_______。
欲除去乙烷中的乙烯，可将混合气体通过盛有_______的洗气瓶；欲除去乙炔中的硫化氢，可将混合气体通过盛有_______的洗气瓶。
浓硝酸常盛装在棕色瓶中，且贮放在冷暗处，原因是_______，
碳酸氢钠溶液和烧碱溶液反应的离子方程式为_______。
常温下，氨水中存在的离子有_______。
在酸或碱存在的条件下，甲酸甲酯跟水发生水解反应的化学方程式为_______。
有下列几种气体：H2、O2、HCl、NO、NH3、CH4，其中通常情况下能用来和水做“喷泉”实验的是_______。
