【单选题】

The amount of sunlight reachingEarth’s surface appears to be growing. The phenomenon, which some dub "global brightening," (1) scientists with a puzzle. If the (2) is real and global, how long will it last and what are the consequences for climate change, the planet’s water cycle, and other (3) that draw energy from sunlight (4) , the answer might seem obvious: More sunlight reaching the ground in a warming world means that temperatures will get warmer (5) . Not so fast, some researchers say.Additional warming would be certain (6) nothing else in the climate system changes.And the climate system is (7) statiC、Some combinations of changes could reinforce the heating; others could (8) it. Unraveling these interactions and forecasting their course require an accurate accounting of the sunlight reaching the surface and the (9) the surface sends skywarD、 Moreover, researchers say, measurements of the sun’s strength atEarth’s surface are potentially powerful tools for (10) human influences on the climate.

Earth’s radiation "budget" (11) an "extremely important parameter that is (12) known,’ says RobertCharlson, an atmospheric scientist at the University of Washington at Seattle. "It needs to be (13) much better than it is."
(14) about the amount of sunlight reachingEarth’s surface were first raised in 1974. Researchers from the United States and Israel recorded a 12% drop (15) sunlight over 40 years at a (16) station in the southern Sinai PeninsulA、 Since then, others have used a variety of techniques to try to track (17) sunlight. Three years ago, for example, a (18) led byBeate Liepert atColumbia University’s Lamont-DohertyEarth Observatory gathered data from ground (19) around the world and found that solar radiation reaching the surface fell (20) 4% from 1961 to 1990.
A．down
B．around
C．to
D．by