The study was the first to tell the difference between 7 and 8 hours of sleep a night（）

A．got to work in a hospital．
B．began to study nursing．
C．started to care for sick people in their homes．
D．became the head of Gentlewomen During Illness．

A. She had one operation when she was six weeks old，which was not successful．
B. While these drugs are given，it is important that the patient does not catch any illness，not even a cold．
C. In this operation a piece of liver, weighing about 250--300 grams，was removed from the father and transplanted into the daughter．
D. David quickly recovered from the operation．
E. However, they are more common in North America and Japan．
F. David was finally able to ride his bike again after about a year．