若﹣7xm+4y2与﹣3x3yn﹣2是同类项，则mn=（ ）。

如图甲所示，一个理想变压器原、副线圈的匝数比n₁：n₂=6：1，副线圈两端接三条支路，每条支路上都接有一只灯泡，电路中L为电感线圈、C为电容器、R为定值电阻．当原线圈两端接有如图乙所示的交流电时，三只灯泡都能发光．如果加在原线圈两端的交流电的峰值保持不变，而将其频率变为原来的2倍，则对于交流电的频率改变之后与改变前相比，下列说法中正确的是（　　）

A．副线圈两端的电压有效值均为6V

B．副线圈两端的电压有效值均为216V

C．灯泡Ⅰ变亮，灯泡Ⅱ变亮

D．灯泡Ⅱ变暗，灯泡Ⅲ变亮

全国第一个志愿者协会成立于（）。

实际上营销，还包括市场研究、客人需要的预测，产品的更新与发展，（）等。

A、推销和服务

B、定价和服务

C、推销和定价

D、定价和推销

Microsoft's COM is a software（1）that allows applications to be built from binary software components. COM is the underlying architecture that forms the foundation for higher-level software services, like those provided by OLE.COM defines a binary standard for function calling between components,a way for components to dynamically discover the interfaces implemented by other components,and a mechanism to identify components and their interfaces uniquely. OLE is a compound（2）standard developed by Microsoft.OLE makes it possible to create（3）with one application and link or embed them in a second application.Embedded objects retain their original format and（4）to the application that created them.Support for OLE is built into the Windows and MacOS Operating Systems.A（5）compound document standard developed mainly by Apple and IBM is called OpenDoC.

空白（2）处应选择（）

A.text

B.graphic

C.document

D.database

液压支架常用乳化油的牌号为（）。

A、M5

B、M10

C、M-T