| 如图所示，两根相距为l的平行直导轨ab、cd、b、d间连有一固定电阻R，导轨电阻可忽略不计；MN为放在ab和cd上的一导体杆，与ab垂直，其电阻也为R；整个装置处于匀强磁场中，磁感应强度的大小为B，磁场方向垂直于导轨所在平面（指向图中纸面内）；现对MN施力使它沿导轨方向以速度v（如图）做匀速运动；令U表示MN两端电压的大小，则（ ） A．U= B．U= C．U=vBl，流过固定电阻R的感应电流由b到d D．U=vBl，流过固定电阻R的感应电流由d到b  |
答案：A
| The idea of test-tube babies may make you either delighted at the wonders of modem medicine or irritated while considering the moral, or legal, or technological implications-of starting life in a laboratory. But if you’ve ever been pregnant yourself, one thing is certain: You wonder what it’s like to carry a test-tube baby. Are these pregnancies normal Are the babies normal The earliest answers come from Australia, where a group of medical experts at the Queen Victoria Medical Center in Melbourne have taken a look at the continent’s first nine successful invitro pregnancies. The Australians report that the pregnancies themselves seemed to have proceeded according to plan, but at birth some unusual trends did show up. Seven of the nine babies turned out to be girls. Six of the nine were delivered by Caesarean section. And one baby, a twin, was born with a serious heart defect and a few days later developed life-threatening problems. What does it all mean Even the doctors don’t know for sure, because the numbers are so small. The proportion of girls to boys is high, but until there are many more test-tube babies no one will know whether that’s something that just happened to be like that or something special that happens when egg meets sperm in a test tube instead of a fallopian tube. The same thing is true of the single heart defect. It usually shows up in only 15 out of 60,000 births in that part of Australia, but the fact that it occurred in one out of nine test-tube babies does not necessarily mean that they are at special risk. One thing the doctors can explain is the high number of Caesareans. Most of the mothers were older, had long histories of fertility problems and in some cases had had surgery on the fallopian tubes, all of which made them likely candidates for Caesareans anyway. The Australian researchers report that they are quite encouraged. All the babies are now making normal progress, even the twin with the birth defects. |
What concern will the test-tube baby raise according to the passage
A．Whether the pregnancies of test-tube babies would be normal.
B．Whether the test-tube babies would be encouraged.
C．Why the proportion of defected babies is so high.
D．Why the number of Caesareans is so high.
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