填空题
[听力原文]11-15
Last year we visited a large modem factory where they make window glass. We first saw workmen mixing sand and other materials together in the right amounts. Then they added some broken glass to the mixture as this helps it to melt. They then fed the mixture into a big hot oven. At the far end of the oven, a stream of liquid glass came out. Here some men lowered a metal frame into the liquid. As the frame came up, it pulled away a hot sheet of glass. Special rollers took hold of the sheet at either side and carried it up wards. 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 glass became cool and another machine cut it into big pieces which workmen stored away together. The glass was now ready for use.