已知x1，x2是方程x2-7x-1=0的两个实数根，则x1+x2=______．

______ he was exercising at the park yesterday morning, he found a purse under the tree.

A. While                           B. Before                            C. Though

用水标法检定标准体积管时，为保证置换器能够以同一速度经过两个检测开关，在一次检定过程中调节阀的开度应（）.

A.前大后小

B.前小后大

C.成比例

D.保持一致

（） an answer, they decided to send an express telegram to them.

A．Received not

B．Having received not

C．Not having received

D．Not received

（）是对一个类中功能一致性的定性度量。

A.组合

B.聚合

C.耦合

D.集合

领导生命周期理论