B.
      At three laboratories in Boston, the world of stem cell research can be captured in all its complexity, promise and diversity. One of the labs focuses on cells taken from human embryos, another on cells from mice and fish, and a third from stem cells that have mysteriously survived in the adult body long after their original mission is over.
      One idea, the focus of about half the stem cell research in the United States, involves studying stem cells that are naturally present in adults. Researchers have found such cells in a variety of tissues and organs and say they seem to be a part of the body’s normal repair mechanism. The other line of research, with stem cells from embryos, has a different obstacle. Although, in theory, the cells could be   16   into developing into any of the body’ s specialized cells, so far scientists are still working on ways to direct their growth in the laboratory and they have not yet effectively cure diseases,even in animals. The most progress with embryonic stem cells is in mice,   17   one group of researchers directed the cells to grow into a variety of blood cells, but not yet the ones they want. Another group directed mouse stem cells to grow into nerve cells and tried to use them to treat Parkinson’s disease in mice. The nerve cells produced the missing chemical,dopamine, but not enough to cure the disease.
      Dr. Bianchi knew that a few fetal cells enter a woman’s blood during pregnancy and discovered that the fetal cells do not disappear when a pregnancy ends. Instead, they remain in a woman’s body for decades, perhaps   18   . If a woman’s tissue or organs are injured, fetal cells from her baby migrate there, divide and turn into the needed cell type. In theory,fetal cells lurking in a woman’s body are the   19   of a new source of stem cells and could be stimulated to treat diseases. But, Dr. Bianchi says, she does not yet know for sure that the cells are stem cells.
      In the basement of a biology building on Harvard’s campus, a small group of scientists is creating human embryos by cloning and obtaining stem cells from those embryos. While many Americans say in polls that they favor using these cells,many others have strong moral objections. Creating and destroying a human embryo to obtain stem cells, they say, is ethically unacceptable, and doing research on human embryonic stem cell lines does not   20   the wrong.The challenge for scientists, in the midst of fierce political debate, many say, is to be realistic about how hard it is to develop treatments.
【題組】16.
(A) coaxed
(B) delved
(C) pummeled
(D) cogitated