短周期元素X、Y、Z、W、Q在元素周期表的位置如表所示，其中X元素的原子内层电子数是最外层电子数的一半，则下列说法正确的是

A．W得电子能力比Q强

B. 由Z与Y组成的物质在熔融时能导电

C. 化合物XQ₄中含有σ键和π键

D．WY₃空间结构为平面三角形

For（1） service, we need a virtual-circuit subnet. Let us see how that works. The idea behind virtual circuits is to avoid having to choose a new （2） for every packet sent. Instead,when a connection is established, a route from the （3）machine to the destination machine ischosen as part of the connection setup and stored in tables inside the（4）That route is used forall traffic flowing over the connection, exactly the same way that the telephone system works.When the connection is released, the virtual circuit is also terminated. With connection-orientedservice, each packet carries an （5）telling which virtual circuit it belongs to.

（3）是（）

A.source

B.route

C.destination

D.host