更多"The more scientists find out about "的相关试题:
[简答题] Scientists have learned more about the way cancer spreads from one place in the body to another. Once it spreads, it gets more difficult to treat. Cancer is a disease of uncontrolled cell growth. Traditional thinking is that cells simply break off the main cancer, travel through the blood and grow in other organs.
But the new findings suggest there is more to this process. Researchers found that cells from the main tumor send out messengers. These prepare the new organ for a secondary tumor (瘤). It works this way. The primary tumor releases proteins called growth factors into the blood. They signal cells at the target organ to produce a sticky protein, called fibronectin. It attaches to the surface of bone marrow (骨髓) cells. The result is a kind of landing area for cancer to arrive and grow into a secondary tumor.
The bone marrow cells help make vessels for blood to pass through and feed the cancer cells. The researchers believe that without the bone marrow cells, the tum
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Scientists are learning more about our
need for sleep. Most people sleep{{U}} (26) {{/U}}eight hours each
night.{{U}} (27) {{/U}}people sleep more than{{U}} (28)
{{/U}}and others sleep as{{U}} (29) {{/U}}as two or three hours each
night. Scientists do not know exactly{{U}} (30) {{/U}}some people sleep
more than others. Dr. Ernest Hartman has a (an){{U}} (31) {{/U}}about
this. He believes that the{{U}} (32) {{/U}}of sleep depends on how a
person{{U}} (33) {{/U}}problems. He said people who need only a few{{U}}
(34) {{/U}}sleep usually are people who have much energy and make good
use of{{U}} (35) {{/U}}to get their work to be done quickly.{{U}}
(36) {{/U}}he said many people who sleep longer than normal do
creative work and seem to need{{U}} (37) {{/U}}dreaming time to find
solution{{U}} (38) {{/U}}emotional problems. Som A. helping B. forcing C. requiring D. allowing
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