单项选择题

Questions 21-25 are based on the following passage:
  Tony Huesman,a heart transplant recipient（接受者）who lived a record 31 years with a single donated organ has died at age 51 of leukemia（白血病）,but his heart still going strong."He had leukemia,"his widow Carol Huesman said,"His heart--believe it or not--held out.His heart never gave up until the end,when it had to."
  Huesman got a heart transplant in 1978 at Stanformed in South Africa.At his death,Huesman was listed as the world's longest survivor of a single transplanted heart both by Stanford and the Richmood,Virgina-based United the end,when it had to."
  Huesman got a heart transplant in 1978 at Stanford University.That was just 11 years after the world's first heart transplant was performed in South Africa.At his death,Huesman was listed as the world's longest survivor of a single transplanted heart both by Stanford and the Richmond,Virginis-based United Network for Organ Sharing.
  "I'm a living proof of a person who can go through a life-threatening illness,have the operation and return to a productive life."Huesman told the Dayton Dail News in 2006.
  Huseman worked as marketing director at a sporting-goods store.He was found to have serious heart disease while in high school.His heart,attacked by a pneunmonia（肺炎）virus，was almost four times its normal size from trying to pump blood with weakened museles.
  Huseman's sister,Landa Huesman Lamb.also was strcken with the same problem and received a heart transplant in 1983.The two were the nation's first brother and sister heart transplant in 1983.The two were the nation's first brother and sister heart transplant recipients.She died in 1991 at age 29.
  Huesman founded the Huesman Heart Foundation in Dayton,which seeks to reduce heart disease by educating children and offers a nursing scholarship in honor of his sister.

The phrase"held out"(Para.1)probably means（）.

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