三、語文填空Passage 1       Cross-species transplantation offers the prospect of an unlimited supply of organs and cells forclinical transplantation, thus resolving the critical shortage of human tissues that currently prohibitsa majority of patients on the waiting list from receiving transplants.   (21)   scientifically asxenotransplantation, transplants of organs and tissues from animals to human actually have a longhistory. There are records of xenotransplantation attempts   (22)  the 17th century. Between the17th and 20th centuries, blood was transfused from various animal species into patients with avariety of pathological conditions. Skin implants were carried out in the 19th century from  (23)  animals, with frogs being the most popular. In the 1920s, Voronoff advocated the transplantation ofslices of chimpanzee testicles into aged men  (24)   "zest for life" were deteriorating, believingthat the hormones produced by the testicles would rejuvenate his patients. Following the pioneeringsurgical work of Carrel,   (25)  , numerous attempts at nonhuman primate organ transplantation inpatients were carried out in the 20th century.
【題組】21.
(A) To know
(B) Known
(C) Knowing
(D) Being known