4. 介词by表示“到……前”。如：职称英语
By Thursday morning you should complete writing the experiment report.
the year 2000 ： 公元2000年。
5. many in high density residential areas ：是独立分词短语。注意它省略了being， 即 many (being) in high density residential areas. 此外，many 指的是 many transmitter towers.
6. at a rate more than three times that of adults 可以理解为 at a rate (that is ) more than three times that of adults， 意为“以比成人多两倍的比率……”短语中的that 指代rate.倍数的表达法是：“数字 times 名词/that”。如：
His salary is three times that of mine.
The sales volume of refrigerators in 2000 was two times that in 1999.
练习：
A. He says there is emerging evidence that children absorb low-level radiation at a rate more than three times that of adults6.
B. By the year 20004 it is estimated that Australia will have 8 million mobile phones： nearly one for every two people
C. “If mobile phones are found to be dangerous， they should carry a warning label until proper shields can be devised，” he said.
D. Then who finances the research?
E. For example， Telstra， Optus and Vodaphone build their towers where it is geographically suitable to them and disregard the need of the community.
F. The conclusion is that mobile phones brings more harm than benefit.
答案与题解：
1. C “空1”应该填入与low-level radiation 有关的内容，例如 low-level radiation 究竟有害还是无害。所以，C是合适的选项。此外，直接阴郁也佐证了选择的合理性。
2. B “空2”前面一句介绍了澳大利亚拥有手机的现状。选项B的内容是对到2000年年底之前手机发展前景的预测，内容连贯，是正确答案。
3. E 这一段的第一句说的是，许多微波发射塔建在人口稠密的居民区。选项E的内容涉及微波发射塔的地点选择只考虑到地理位置，而不顾及公众的需求。选项E扩展了第一句表达的信息。此外，选项E中出现 tower 这个词，与第一句的 tower 相呼应，也佐证了选择的合理性。
4． A “空4”前面一句说的是Robert Bell建议政府应该禁止在学校操场、儿童日托中心、医院、运动场所以及儿童占比例较高的居住场所方圆500米范围内建发射塔。选项A说明要这样做的原因，因而是答案。
5. D “空5”后面一句说的是 major telephone companies 出钱资助研究项目的问题，提示了D是正确答案。