通读下面的短文，掌握其大意。然后，从每小题的四个选项中选出最佳选项。
Enough sleep is important to health. The amount of sleep needed 21 on the age of the person and the conditions in which sleep 22 . The young may need more sleep than the old, but 23 eight hours are enough for the health of grown-ups. Some can do with less than this amount, but 24 may need more. Every person knows his own need. It is then a matter of how to 25 it. Sleep should be always enough to make one bring back his strong and get ready for 26 work.
Fresh air is 27 to sound sleep. So it is not 28 reason for some people to think that it is practicable to sleep in the open air. 29 a person can keep himself warm, out-of-door sleeping probably gives the body 30 complete relaxation.
Ability to sleep is largely a habit. Out-of-door 31 , a good habit of regular drinking and the avoidance of late eating and 32 are all helpful to sound sleep. Such factors are largely within the 33 of any person. A bath at 34 , neither hot nor cool but of body temperature, may be helpful to sleep. Sleep producing drug（帮助睡眠的药物）should never be taken except when suggested by 35 .
____.
通过上句话中的grown-ups，可推断出本题的答案是children。
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_______。
