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【单选题】
20钢属于优质碳素结构钢中的（ ），其强度低，但塑性和韧性好，可加工性和焊接性能优良。A.
弹簧钢
B.
渗碳钢
C.
调
D.
模具钢
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举一反三
【单选题】It for four days when he arrived, so the roads were very muddy.
A.
was raining
B.
would be raining
C.
had been raining
D.
has rained
【单选题】Was your journey to Hawaii pleasant last summer? No, it _________ for one week when we arrived, so the roads were very muddy.
A.
has rained
B.
was raining
C.
has been raining
D.
had been raining
【单选题】若凸轮轮廓作用角为2φ,差动角为2αs,为了保证喷油泵换向后供油提前角β不变,则必须满足下列关系 。
A.
2φ = αs –β
B.
2αs = 2φ - 2β
C.
2φ = 2φ - 2β
D.
2αs = 2φ + 2β
【简答题】Muddy has been _______ a golf scholarship at the University of Hawaii.
【单选题】若凸轮轮廓作用角为2φ，差动角为2α S ，为了保证喷油泵换向后供油提前角β不变，则必须满足下列关系（）
A.
2φ=α S －β
B.
2α S =2φ－2β
C.
2φ=2α S －2β
D.
2α S =2φ＋2β
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