第 16 題至第 20 題為篇章結構題組，下列各題請依文意從四個選項中選出最合適者，答案選用不能重覆。
Regenerative medicine is a field with big ambitions. It hopes, one day, to repair or replace worn-out hearts, livers,
kidneys and other vital organs. Many people, though, would settle for a humbler repair—of their teeth.
16 But if work by people such as David Mooney of Harvard University comes to fruition, the days of drill and
fill may be numbered. Dr. Mooney and his team have found a surprising way to get dentine, the tissue that underlies a
tooth’s enamel coat, to repair itself. They do so by shining a laser beam at it.
Regenerative medicine boils down to the intelligent manipulation of stem cells. A stem cell is one that has the capacity
to split asymmetrically so that one remains a stem cell while the other gives birth to a line which proliferates and
differentiates into many other sorts of cell. The most famous and controversial stem cells are those in early
embryos. 17 Mature tissues such as dentine contain stem cells of more limited capacity, which keep up a
supply of new specialized cells to replace old ones as they die.
Dr. Mooney’s trick is to tickle dentine’s stem cells in a way that encourages them to proliferate and produce more
dentine. 18 The light it shines creates chemically potent, oxygen-rich molecules such as hydrogen peroxide
which go on to activate latent versions of molecules called transforming growth factor-beta 1 (TGF-beta 1).
19 This is a preliminary result, and it does not address the question of whether enamel might similarly be
repaired. 20 Eventually, perhaps, dentists will approach cavities with lasers rather than drills—and the days of
fillings will be over. 【題組】 20 (A) But it is encouraging. (B) But it is discouraging.
(C) But it subverts the previous results. (D) But it proves nothing.
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