阅读理解（第3l～45题，每题3分，共15分）
请根据短文内容，为每题确定1个最佳选项。
Human Heart Can Make New Cells
Solving a longstanding mystery, scientists have found that the human heart continues to generate new cardiac cells throughout the life span, although the rate of new cell production slows with age.
The finding, published in the April 3 issue of Science, could open a new path for the treatment of heart diseases such as heart failure and heart attack ,experts say.
"We find that the beating cells in the heart, cardiomyocytes, are renewed, " said lead researcher Dr. Jonas Frisen, a professor of stem cell research at the Karolinska Institute in Stockholm, Sweden. "It has previously not been known whether we were limited to the cardiomyocytes we are bom with or if they could be renewed." he said.
The process of renewing these cells changes over time, Frisen added. In a 20-year-old, about 1 percent of cardiomyocytes are exchanged each year, but the turnover rate decreases with age to only 0. 45 percent by age 75.
"If we can understand how the generation of new cardiomyocytes is regulated , it may be potentially possible to develop pharmaceuticals that promote this process to stimulate regeneration after for example, a heart attack, " Frisen said.
That could lead to treatment that helps restore damaged hearts.
"A lot of people suffer from chronic heart failure," noted co-author Dr. Ratan Bhardwaj.
Ratan Bhardwaj, also from the Karolinska Institute. " Chronic heart failure arises from heart cells dying, "he said.
With this finding, scientists are "opening the door to potential therapies to having ourselves heal ourselves, " Bhardwaj said. " Maybe one could devise a pharmaceutical agent that would make heart cells make new and more cells to overcome the problem they are facing."
But barriers remain. According to Bhardwaj , scientists do not yet know how to increase heart cell production to a rate that would replace cells faster than they are dying off, especially in older patients with heart failure. In addition, the number of new cells the heart produces was estimated using healthy hearts-whether the rate of cell tumover in diseased hearts is the same remains unknown.
31. The human heart stops producing cardiac cells
A. when a person becomes old
B. as soon as a person gets sick
C. immediately after a person is born
D. once a person dies
32. The finding could prove to be useful to
A. the analysis of cardiac cells
B. the prevention of chronic diseases
C. the treatment of heart disease
D. the study of longstanding mysteries
33. In people in their mid-70s, only 0. 45 percent of cardiomyocytes _ .
A. are still functional
B. are reduced each year
C. are replaced each year
D. are damaged each year
34. Chronic heart failure is attributed to
A. the dying heart cells
B. the effect of pharmceutical
C. the weight of a patient
D. the life span of a person
35. It is not known yet if the rate of cell turnover in diseased hearts
A. is high enough to replace cells faster than they're dying off
B. is of any use to researchers
C. is the same as that in healthy hearts
D. changes over time
参考答案及解析
31.D [解析]根据是第一段中：…the human heart continues to generate new cardiac cells throughout the life span。在人的整个生命进程中，其心脏不时产生新的心肌细胞。言外之意，这个过程直到人死才会停止。
32.C [解析]根据是第二段中：The finding，…，could open a new path for the treatment of heart diseases 这一研究成果将为心脏病的医治开拓新的途径。
33.C[解析]第四段说，一个人20岁时，其每年被更替的心肌细胞为1%，随着年龄的增长，该更替率( turnover rate)不断下降，等到75岁时，只有0.45%。
34.A [解析]倒数第三段说：Chronic heart failure arises from heart cells dying。慢性心衰是由心肌细胞死亡导致的。
35.C[解析]最后一句说：心脏产生的新细胞的数量是按照健康心脏计算的，而有心脏病的细胞更替率是否与健康心脏相同还不得而知。
