气动执行机构在安装上有哪些要求？

—Did you find the missing couple in the mountain yesterday?

—No,but we _____ to get in touch with them ever since.[ ]

A. have tried  

B. have been trying

C. had tried  

D. had been trying

Jerry had worked hard for his chemistry examination so that he could be sure of passing it at his first_____ .

A．assumption

B．appetite

C．attempt

D．accumulation

国际标准音调的频率是440Hz，它的物理意义是______．

一定温度时，向2.0 L恒容密闭容器中充入2 mol SO₂和1 mol O₂，发生反应：

2SO₂(g)＋O₂(g) 2SO₃(g)。经过一段时间后达到平衡。反应过程中测定的部分数据见下表：

下列说法正确的是（  ）

A．反应在前t₁s的平均速率v(O₂)＝0.4/t₁ mol·L^－1·s^－1

B．保持其他条件不变，体积压缩到1.0 L，平衡常数将增大

C．相同温度下，起始时向容器中充入4 mol SO₃，达到平衡时，SO₃的转化率大于10%

D．保持温度不变，向该容器中再充入2 mol SO₂、1 mol O₂，反应达到新平衡时n(SO₃)/n(O₂)增大

通过绩效诊断找出绩效低下的原因，管理者充当导师帮助员工克服障碍提高绩效的过程是( )。

A．绩效监控
B．绩效考核
C．绩效辅导
D．绩效改进