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问题 单项选择题

( )含量增加，可引起钢材的“热脆性”，会降低钢材的各种机械性能，使钢材的可焊性、冲击韧性、耐疲劳性和抗腐蚀性等均降低。

A．硫
B．磷
C．氧
D．氮

答案

参考答案：A

单项选择题

A．The speaker’s friend entered his house from back door.

B．The speaker’s friend entered his neighbor’s house by mistake.

C．A thief entered the speaker’s house by mistake.

D．The speaker’s friend entered the room by climbing the apple tree.
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单项选择题

毛毛骑在牛背上过河，他共有甲、乙、丙、丁4头牛，甲过河要20分钟，乙过河要30分钟，丙过河要40分钟，丁过河要50分钟。毛毛每次只能赶2头牛过河，要把4头牛都赶到对岸去，最少要多少分钟

A．190
B．170
C．180
D．160
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