下列隧道协议中工作在网络层的是（ ）。

(单选题)They believed that by using computers the production of thei (单选题)The matter ( ) your study surely requires ( ) carefull (单选题)I don’t like ( ) bills but when I do get them I like ( (单选题)Fred entered without knocking and, very out of breath, sank (单选题)He wasn’t asked to take on the chairmanship of the society, (单选题)I think he is taking an active part in social work. I agree (单选题)We realized that he was under great ( ), so we took no no (单选题)I hope my school life of senior high will be ( ) than tha (单选题)After the new technique was introduced, the factory produced (单选题)She ( ) fifty or so when I first met her at a conference.