选用焊条种类时，不必考虑焊件材料的（）。
A.力学性能
B.化学成分
C.受力情况
D.表面质量
参考答案：D
设f’(x)在[0，+∞)上单调减少，且
证明级数
收敛。
— Do you still remember the first time we met?
—Of course. You _____ an English evening party.[ ]
A. organized
B. have organized
C. had organized
D. were organizing
收敛。