The underlined word "they" ( paragraph 5 ) refers to ______.

A．The theory of programmed cell deaths.
B．A great scientist-Sir John Sulston.
C．The programmed human life.
D．Dangerous diseases.

A．found that human beings are similar to the worm
B．got the fact we share 40 percent of our genetic structure with the simple worm
C．found the computer which controls each of the cells in the human body
D．proved that cell death is programmed