结构推理 1 Last week, we visited a large factory where they make window glass.
   ______ Special rollers took hold of the sheet at either side and carried it upwards.
   ______ Here some men lowered a metal frame into the liquid.
   ______ They then fed the mixture into a big hot oven.
   ______ Our guide told us that at this stage it was necessary for the glass to cool slowly as this would make it very strong.
   ______ At a height of 30 feet, the sheet of the glass became cool and another machine cut it into big pieces which workmen stored away together.
   ______ Then they added some broken glass to the mixture as this helps it to melt.
   ______ We first saw workmen mixing sands and other materials together in the right amounts.
   ______ At the far end of the oven, a stream of liquid glass came out.
   ______ As the frame came up, it pulled away a hot sheet of glass.
    11 Finally the glass was ready for use.