证券投资基金有哪些特征？它与股票、债券有哪些区别？
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材料的导热性和保暖性分别指什么，举例说明静止空气和水对于材料保暖性的影响。
带二极管整流器的SPWM变频器是以正弦波为逆变器输出波形，是一系列的（）矩形波。
A. 幅值不变，宽度可变
B. 幅值可变，宽度不变
C. 幅值不变，宽度不变
D. 幅值可变，宽度可变
You have created a script in the recovery catalog called backup_database. Which of the following commands would successfully execute that script？（）
A. run { open script backup_database； run script backup_database }
B. run { engage script backup_database； }
C. run { run script backup_database； }
D. Run { execute script backup_database； }
E. The name backup_database is an invalid name for an RMAN script. Trying to run it from RMAN would result in an error.
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