I am watching my son and hundreds of his classmates, marching in perfect order into the university football stadium（体育场）. Holding back my tears, I search for my son in the sea of people. Suddenly, there he is. I jump to my feet and start waving. Arms flying back and forth above my head, l look like a windmill（风车）, or maybe an airport worker guiding a plane. I see him smile, then laugh as he pulls his camera out of his pocket. He takes a picture of me as he waves back, and then goes to find his seat.
What is it about graduations that makes them so moving? Why do most of us women cry as we watch our sons, daughters, grandchildren, nieces , and nephews march in dark robes（长袍）? We cry partly because, no matter how old our children are, and no matter how much they have achieved, in our eyes they are still the little babies we carry around the house to comfort.
And, of course, we cry also because something wonderful is ending, and we miss it already. Whatever is on its way to take its place will not be the same. It has been such a gift to have my son go to college close to home. We never have to worry about whether he will be able to make the trip home for a holiday. But this was a time-limited gift. My son will attend graduate school in California. We will talk and text like crazy, and visit as often as we can. But it will not be the same.
Why are people gathering in the university stadium?（ ）
事实细节题。由第二段可知，学生和家长们聚集在体育场是为了参加毕业典礼。故C正确。
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_______。
