洞式渗透适用于地下水流量较大的地段，洞壁宜采用（）砌筑。

A．浆砌片

B．盖板

C．混凝土

D．黏土

Almost everyone likes dogs, and almost everyone likes to read stories about dogs.
I have a friend. He has a big police dog named Jack. Police dogs are very clever. Every Sunday afternoon my friend takes the dog for a long walk in the park. Jack likes these long walks very much.
One Sunday afternoon, a young man came to visit my friend. He stayed a long time. He talked and talked. Soon it was time for my friend to take Jack out for a walk. But the visitor still stayed. Jack became very worried. He walked around the room several times and then sat down in front of the visitor and looked at him. But the visitor paid no attention. He went on talking. At last Jack got angry. He went out of the room and came back a few minutes later. He sat down again in front of the visitor, but this time he took the visitor’s cap in his mouth.

Jack sat down in front of the visitor because he wanted ______.

A. to see the visitor
B. to show the visitor how clever he is
C. the visitor to leave