燃气管道材料应根据系统压力和敷设条件选用。一般情况下，高压系统可以采用( )。
A．铸铁管
B．聚乙烯管
C．钢管
D．合金钢管
参考答案：C
根据所给的句子，仿照例句用must或mustn't造句。
e.g. Never swim in the waterfall.
You mustn't swim in the waterfall.
1. Never speak loudly in the library.
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2. Never sleep outside at night in winter.
3. Don't play with fire.
4. Always do warm-up exercises before running.
5. Always wear sports shoes on Sports Day.
风景伐
