In recent years, remote-sensing technologies have become ordinary in archaeological fieldwork（实地考察）. Such tools for excavation produce rapid results and cause no damage to archaeological sites. They are highly accurate and usually cost effective. Here are three of the modern archaeologist's most trusted remote-sensing tools.

    As the simplest of the remote-sensing techniques that archaeologists use, aerial（空中的）photography allows experts to see aspects of a site that may be invisible from the ground, such as the way in which something such as a town, garden, or building is arranged and traces of old walls and roads. The technique involves taking photographs with conventional cameras and filming from airplanes, helicopters, hot-air balloons, or other airborne vehicles.

    Geographic Information System (GIS) contains a large amount of field data archaeologists typically collect in and around excavation sites. While in the field, archaeologists use GIS on their computers to make and manage detailed site maps, and they can combine the results of remote-sensing tests with maps of the region created with the aid of Global Positioning System. Resulting maps sort the most archeologically promising areas and display these sites three-dimensionally.

    Ranging in size from small handled models that one places against the ground to larger ones that one across a site, Ground Penetrating Radar (GPR) devices use low-power radio waves to detect changes underground. Unlike traditional radar, which broadcasts into the air and uses a dish to focus the returned waves, GPR uses a small but sensitive receiver placed directly against the ground. Depending on their needs, archaeologists can adjust radio frequencies upward for shallow sites or downward for deeper areas, though GPR devices produce the greatest definition（清晰度）when reading depths of three feet or less.

在“探究凸透镜成像规律”的实验中，当凸透镜、光屏和蜡烛的位置如图所示时，光屏上能成清晰的像，那么（　　）

1）陈丽的个性特点以及专长。

2）陈丽平时在校的表现(她的学业表现和获奖情况等)。

注意：1）词数100左右。2）可以适当增加细节，以使行文连贯。3）开头语和结束语已为你写好。

Dear Chairman,

    I am Li Hua, Chen Li's classmate and also chairman of the Students' Union. …

Yours sincerely,

Li Hua

    Every day, a homeowner in Pennsylvania is reminded of a mistake he made 13 years ago. At around 8 in the evening, his peaceful house comes alive with the noise of an alarm clock he dropped into the wall over ten years ago. It's been stuck in the wall ever since.

    13 years ago, Jerry Lynn wanted to make a hole through his living room wall, so he could pass the television cable (电缆)through it. He didn't want to destroy any pipes in the wall, so he came up with a good idea to make sure that he was drilling(钻孔) in an empty place. He took his alarm clock, tied it to a string(线) and set it to go off in 10 minutes. Then he lowered it into the wall, and waited for it to ring. His goal was to drill a hole in a safe place near the place where the ringing was coming from, making sure that there was nothing to pass through there. It all went smoothly until the alarm clock fell into the wall.

The man didn't get back the alarm clock, but thought it would probably run out of battery(电池)in several months, so he wasn't too worried about it. But 13 years later, the alarm clock still rings every day.

    As you can imagine, Jerry and his wife have gotten used to the ringing living room wall, but it can still surprise  guests. So Jerry decides to pull down that wall, just to stop that boring thing, though after putting up with it for 13 years, he think it will be a pity. He says, “I've never herd of a clock battery lasting that long and my friends think it may be interesting to see how long it will keep ringing for.”