C.the way the families lived in the communities

D.the way the participants buried themselves

4.The writer's attitude towards social media is 　　　.

A.ambiguous B.positive

C.unknown D.negative

　　Shiny things absorb less heat when left in the sun. This means that if the Earth could be made a little shinier it would be less likely to suffer global warming. Ways to brighten it, such as adding nanoscale(纳米级)specks of salt to low clouds, making them whiter, or putting a thin haze of particles into the stratosphere(平流层), are the field of "geoengineering"(地球工程). A small band of scientists have mostly been using computer models to study the subject. Some of them are now proposing outdoor experiments-using seawater-fed sprayers to churn out particles of the exact size needed to brighten clouds, or scattering sulphur particles(硫粒子) from underneath a large balloon 20km up in the sky.

　　The scientists hope to understand some of the processes on which these technologies depend, as a way of both measuring their possibility (can you reliably make the proper amount of sea salt brighten clouds?) and assessing their risks (how much damage to the ozone layer might a stratospheric haze do, and how might such damage be minimized?). The experiments would be far too small to have any climatic effects. The amount of sulphur put into the stratosphere by the experimental balloon would be 2% of what a passenger jet crossing the Atlantic emits in an hour.

　　However, many scientists doubt whether geoengineering experiments have any effects. And some environmentalists say that such experiments reflect the hubris(傲慢)of humans, who believe they can toy with nature.

Geoengineering is not an alternative to relieving climate change by cutting carbon emissions. Even if emissions do start to fall, the cuts will take decades to have any effect so temperatures are likely to go on rising for some time. The planet is not getting cooler and the pressures on the climate are unlikely to go away. It is therefore not too hard to imagine a world, decades from now, in which emissions are falling but temperatures are rising steeply and the ability to adapt to them has been stretched too far. An additional way to stabilize temperatures might then seem appropriate. Geoengineering offers that possibility.