室内供暖管网系统的布置一般遵循的原则有（）。

A.散热器通常安装在窗的下部，以便使散热器表面上升的热空气与冷风混合而降低温度后再循环至室内中部，以此来抵御冷风侵入和增加对人的舒适性

B.立管一般布置在窗间靠近墙处或墙壁的角落

C.位于顶层顶棚和地沟内的干管有保温措施，以减少热损失和防止冻结

D.根据供暖系统的形式布置干管，一般合理地分成若干支路，以便阻力损失平衡

E.供暖管道系统的引入口一般设置在建筑物热负荷对称的位置，大多是建筑物的上部，以系统作为半径

The idea of test-tube babies may make you either delighted at the wonders of modern 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.

Which of the following statements about the experiment mentioned in the passage is true

A．Only the twins are defected.

B．Most of the babies are delivered by means of Caesarean.

C．There are some troubles during all mothers’ pregnancies.

D．One baby appears to be abnormal.