Lisa was running late．Lisa,25, had a lot to do at work, plus visitors on the way : her parents were coming in for Thanksgiving from her hometown．But as she hurried down the subway stairs, she started to feel uncomfortably warm．By the time she got to the platform,Lisa felt weak and tired--maybe it hadn't been a good idea to give blood the night before,she thought．She rested herself against a post close to the tracks．
Several yards away,Frank,43, and his girlfriend,Jennifer, found a spot close to where the front of the train would stop．They were deep in discussion about a house they were thinking of buying．
But when he heard the scream, followed by someone yelling, " Oh, my God, she fell in!" Frank didn't hesitate．He jumped down to the tracks and ran some 40 feet toward the body lying on the rails．"No!
Not you!" his girlfriend screamed after him．
She was right to be alarmed．By the time Frank reached Lisa, he could feel the tracks shaking and see the light coming．The train was about 20 seconds from the station．
It was hard to lift her．She was just out．But he managed to raise her the four feet to the platform so that bystanders could hold her by the arms and drag her away from the edge．That was where Lisa briefly regained consciousness ,felt herself being pulled along the ground,and saw someone else holding her purse．
Lisa thought she'd been robbed．A woman held her hand and a man gave his shirt to help stop the blood pouring from her head．And she tried to talk but she couldn't, and that was when she realized how much pain she was in．
Police and fire officials soon arrived, and Frank told the story to an officer．Jennifer said her boyfriend was calm on their 40-minute train ride downtown--just as he had been seconds after the rescue,which made her think about her reaction at the time． "I saw the train coming and I was thinking he was going to die," she explained．
What was the most probable cause for Lisa's weakness?
事实细节题。从第一段倒数第二句可知，Lisa怀疑自己虚弱的原因是她献血了。
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_______。
