题目内容：
通读下面的短文，回答题。 Lisa was running late． Lisa, 25, had a lot to do at work, plus visitors on the way:her parentswere coming in for Thanksgiving from her hometown． But as she hurried down the subway stairs, shestarted 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 herselfagainst a post close to the tracks．
Several yards away,Frank,43 ,and his girlfriend,Jennifer,found a spot close to where the frontof 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!" Frankdidn't hesitate． He jumped down to the tracks and ran some 40 feet toward the body lying on therails． "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 frome station．
It was hard to lift her． She was just out． But he managed to raiseher 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 Lisabriefly regained consciousness, felt herself being pulled along the ground, and saw someone elseholding her purse．
Lisa thought she'd been robbed． A woman held her hand and a man gave his shirt to help stopthe blood pouring from her head． And she tried to talk but she couldn't, and that was when sherealized how much pain she was in．
Police and fire officials soon arrived, and Frank told the story to an officer． Jennifer said herboyfriend was calm on their 40-minute train ride downtown--just as he had been seconds after therescue,which made her think about her reaction at the time． "I saw the train coming and I was thinkinghe was going to die," she explained．
What was the most probable cause for Lisa's weakness? A．She had run a long way．
B．She felt hot in the subway．
C．She had done a lot of work．
D．She had donated blood the night before．
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答案解析：