Listening Device Provides Landslide(山崩；地滑；塌方)Early Warning
A device that provides early warning of a landslide by monitoring vibrations (振动)in soil is being tested by UK researchers. The device could save thousands of lives each year by warning when an area should be evacuated(疏散，撤走), the scientists say. Such natural disasters are common in countries that experience sudden, heavy rainfall(降雨，降雨量), and can also be triggered by earthquakes and even water erosion(腐蚀，侵蚀).
Landslides start when a few particles of soil or rock within a slope start to move, but the early stages can be hard to spot. Following this initial movement, "slopes can become unstable in a matter of hours or minutes, " says Nell Dixon at Southborough University, UK. He says a warning system that monitors this movement "might be enough to evacuate a block of fiats or clear a road, and save lives. "
The most common way to monitor a slope for signs of an imminent (即将发生的) landslide is to watch for changes in its shape. Surveyors can do this by measuring aside directly, or sensors(传感器) sunk into boreholes(钻孔，井眼)or fixed above ground can be used to monitor the shape of a slope. Slopes can, however, change shape without triggering a landslide, so either method is prone to causing false alarms. Now Dixon's team has developed a device that listens for the vibrations' caused when particles begin moving within a slope.
The device takes the form. of a steel pipe dropped into a borehole in a slope. The borehole is filled in with gravel(砾，沙砾，砾石)around the pipe to help transmit high-frequency vibrations generated by particles within the slope. These vibrations pass up the tube and are picked up by a sensor on the surface. Software analyses the vibration signal to determine whether a landslide may be imminent.
The device is currently being tested in a 6-metre-tall artificial clay embankment(堤岸)in Newcastle, UK. Early results suggest it should provide fewer false positives than existing systems. Once it has been carefully and thoroughly tested, the device could be used to create a complete early-warning system for dangerous slopes.
"Locations with a significant risk of landslides could definitely benefit from a machine like this, " says Adam Poulter, an expert at the British Red Cross. "As long as it doesn't cost too much, " But, Poulter adds that an early-warning system may not he enough on its own. "You need to have the human communication, " he says. "Making systems that get warnings to those who need them can be difficult. "
What does "Such natural disasters" in the first paragraph refer to？

A. Sudden, heavy rainfall.
B. Earthquakes.
C. Water erosion.
D. Landslides.