问题
选择题
Mr. Black has been in Beijing a week. [ ]
A. since
B. for
C. after
D. before
答案
答案：B
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Mr. Black has been in Beijing a week. [ ]
A. since
B. for
C. after
D. before
答案：B
20（As=941mm2），梁的混凝土保护层厚度c=25mm。
；假定σs=285N/mm，则该梁的短期刚度Bs最接近于（）N/mm2。