Question

The recent crash landing （強(qiáng)行著陸）of Russian spacecraft Phobos-Grunt has focused attention on the increasing amount of space junk orbiting the planet. So does this mean the Earth has been getting lighter? The BBC's Radio 4 programme More or Less turned to a group of Cambridge University academics for the answer.
   There are factors that are causing Earth to both gain and lose mass(物體的質(zhì)量) over time, according to Dr Chris Smith, a medical microbiologist and broadcaster who tries to improve the public understanding of science.
    By far the biggest contributor to the world's mass is the 40,000 tonnes（公噸） of dust that is falling from space to Earth a year, says Dr Smith. "The Earth is acting like a giant vacuum cleaner（真空吸塵器）powered by gravity in space, pulling in particles of dust,"
   Another much less significant reason the planet is gaining mass is because of global warming.

"NASA has calculated that the Earth is gaining about 160 tonnes a year because the temperature of the Earth is going up. If we are adding energy to the system, the mass must go up," says Dr Smith.
    This means that in total between 40,000 and 41,000 tonnes is being added to the mass of the planet each year.
   Population growth and new buildings are not a factor, he says, because both of these are actually made up of existing matter on the planet.
   But overall, Dr Smith has calculated that the Earth - including the sea and the atmosphere - is losing mass. He points to a handful of reasons.
   For instance, the Earth's core （核心）is like a giant nuclear reactor （反應(yīng)堆）that is gradually losing energy over time, and that loss in energy translates into a loss of mass. But this is a tiny amount - he estimates no more than 16 tonnes a year.
    But there are some gases such as hydrogen escaping from the atmosphere. Dr Smith says, " The Earth is losing about three kilograms of hydrogen gas every second. It's about 95,000 tonnes every year.”
   "The other very light gas this is happening to is helium（氦）and there is much less of that, so it's about 1,600 tonnes a year of helium that we lose."

72. We can learn from the text that Phobos-Grunt most probably _________.

   A. faced dangers when travelling in space             B. was forced to land by a terrorist

   C. bumped into a satellite                          D. exploded while taking off

73. What can we know about Dr Chris Smith?

   A. He is more interested in broadcasting than science.

   B. He is one of the broadcasters of BBC’s Radio 4.

   C. He never thought the earth would become light.

   D. He hopes more people can understand science

74. The underlined word “this” in the last paragraph refers to the fact that_______.

   A. the weight of the Earth is changing                

B. gases escape from the atmosphere  

   C. dust is falling from space to the Earth              

D. the Earth is getting even warmer

75. We can learn from the text___________.

   A. the Earth lost much less energy in the past 

B. humans can sense the weight loss of the Earth

   C. the Earth will become heavier in the near future

D. the Earth can suck up the particles of dust in space

Answer 1

  ADBD