3、A summary of the physical and chemical nature of life must begin，not on the earth but in the sun；in fact，at the sun’s very center．It is here that is to be found the source of the energy that the sun constantly pours out into space as light and heat．This energy is liberated at the center of the sun as billions upon billions of nuclei hydrogen atoms clash with each other and fuse together to form nuclei of atoms．The output of light and heat of the sun requires that some 600 million tons of hydrogen be converted into helium(氦)in the sun every second．Thus the sun has been doing for several thousands of millions of years．

      The nuclear energy is released at the sun’s center as high-energy gamma radiation, a form of electromagnetic(電磁的)radiation like light and radio waves，only of very much shorter wavelength．This gamma radiation is absorbed by atoms inside the sun，to be remitted at slightly longer wavelengths．This radiation in its turn，is absorbed and remitted．As the energy filters through the layers of Solar inner，it passes through the X-ray part of the spectrum(光)，eventually becoming light．At this stage，it has reached what we call the Solar surface，and can escape into space，without being absorbed further by solar atoms．A very small fraction of the sun’s light and heat is given off in such direction that，after passing unhindered through interplanetary space，it hits the earth．

1．What does the passage mainly discuss?

    A．The production of solar light and heat．

    B．The physical and chemical nature of life．

    C．The conversion of hydrogen to helium．

    D．Radiation in the X—ray part of the spectrum．

2．The passage indicates that，in comparison to radio waves，gamma waves _________．

      A．produce louder sound                                B．a(chǎn)re less magnetic

      C．do not form in the sun’s center                           D．a(chǎn)re not so long

3．According to the passage，through which of the following does the energy released in the sun pass before it becomes light?

      A．The X-ray part of the spectrum．               B．Electromagnetic space．   

      C．The Solar surface．                                   D．Interplanetary space．