Passage Three
The war had begun, and George had joined the air force. He wanted to be a pilot and after some months he managed to get to the air force training school, where they taught pilots to fly.
There, the first thing that new students had to do was to be taken up in a plane by an experienced pi lot, to give them some ideas what it felt like. Even those who had traveled as passengers in commercial (商业的) airline planes before found it strange to be in the cockpit (驾驶舱)of a small fighter plane, and most of the students felt nervous.
The officer who had to take the students up for their first flight allowed them to fly the plane for a few seconds if they wanted to and if they were not too frightened to try, but he was always ready to take over as soon as the plane started to do dangerous things.
George was one of those who took over the controls of the plane when he went up in it for the first time, and after the officer had taken them from him again. George thought that he had better ask a few questions to show how interested he was and how much he wanted to learn to fly. There were a number of instruments (仪表) in front of him, so he chose one and asked the officer what it was. The officer looked at him strangely for a moment and then answered, "That is the clock."
The officer allowed（）
A．the students to fly the plane all the time
B．all students to fly the plane
C．the students who were not frightened to fly on their own
D．some students to fly a little while
根尖部的龈黏膜呈半球形隆起，不红，时有乒乒球感，富有弹性，穿刺内容呈黄色透明液体。