Florence Nightingale was 小题1:b_____ on 12th May, 1820. In 1851 she trained 小题2:a_____ a nurse. Two years 小题3:l_____ she went to a British military hospital so that she 小题4:c_____ help British soldiers. Before she arrived, many soldiers died in the hospital because it was dirty. Nightingale worked 小题5:.h_____ to make the hospital 小题6:c_____ and stop soldiers 小题7:d_____. When she returned to England in 1856, she was a national heroine. Nightingale wrote two books about nursing and she 小题8:a_____ started a school to 小题9:t_____ nurses. She 小题10:d_____ in London on 13th August, 1910, when she was 90 years old.

说明随机变量X的方差D（X）的意义。