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【单选题】
Read the following paragraph from the introduction [paragraphs 1-5] of the article. Hide-and-seek is hard. It is harder than something like playful wrestling. You have to know the rules to play hide-and-seek. You also need to understand different roles. What is the focus of this paragraph?A.
why hide-and-seek is hard
B.
why playful wrestling is hard
C.
what the rules of hide-and-seek are
D.
what the roles of hide-and-seek are
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举一反三
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A.
离子气和钨极不反应
B.
保护气和母材不反应
C.
大电流焊接时，一般离子气与保护气成分相同
D.
氩气和氢气的混合气体可以用于焊接钛、锆等活泼金属
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