填空题
What sort of factory did we visit last year?
Last year we visited a ___________.
填空题
Where did workmen put t he mixture of sand and other materials?
They fed the mixture into ___________.
填空题
Where did some workmen lower a metal frame into?
They lowered the metal frame into ___________.
填空题
Why is it necessary for glass to cool slowly?
Because this would ___________.
填空题
What does this passage mainly discuss?