研究物质的变化时可使用具有放射性的¹⁸O （这种氧原子核内有8个质子和10个中子）作为“示踪原子”．科学家希尔和尼尔研究证明，光合作用的通式应更合理地表达为：6CO₂+12H₂¹⁸O

A．在植物体内的淀粉和蛋白质中发现¹⁸O

B．在植物体内的葡萄糖（C₆H₁₂O₆）中发现¹⁸O

C．在植物体周围的空气中发现¹⁸O

D．无法确定¹⁸O在什么地方出现

For（）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（）for every packet sent. Instead, when a connection is established, a route from the（）machine to the destination machine is chosen as part of the connection setup and stored in tables inside the（）That route is used for all 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-oriented service, each packet carries an（）telling which virtual circuit it belongs to.

The idea behind virtual circuits is to avoid having to choose a new（）for every packet sent.

A．processor

B．device

C．route

D．terminal