单项选择题
A．no plants would grow on our planet B．earth would be thirty degrees colder C．man could not live on the earth D．all of the above answers are correct
A．the way in which gases are used for keeping plants warm B．a natural course which is to some degree helpful to us C．the way" greenhouse gases trap heat on the earth D．the whole course in which greenhouse gases prevent heat
A．Burning too much coal and oil produces lots of greenhouse gases. B．It has become warmer on the earth now than in the past. C．Gases put into the atmosphere now will affect the earth years later. D．The temperature in a greenhouse is as high as that in the atmosphere.