Questions 18 to 20 are based on the following monologue about rainwater. You now have 20 seconds to read Questions 18 to 20. What is the main topic of the talk
A. How quartz sand is formed.
B. How underground water differs.
C. How rain is formed.
D. How water tables change.
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Questions 18 to 20 are based on the following monologue about rainwater. You now have 20 seconds to read Questions 18 to 20. What characteristics of hard water does the speaker mention
A. It is undrinkable.
B. It has minerals in it.
C. It is slightly colored.
D. It only tastes horrible.
采用交流电时，因极性不断地变化，所以磁偏吹现象更为严重。( )
A. 对
B. 错
当焊条电弧焊弧长增加时，电弧静特性曲线上移。 ( )
A. 对
B. 错
中厚板焊接时，应该采用多层焊或多层多道焊。( )
A. 对
B. 错
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