用所给动词的正确形式填空
It is four o’clock in the afternoon. Some students ________ ________ ( play) basketball on the playground . They  ________ ________ (not go ) home until it is dark.
Lin Tao ________ (be) a scientist now. He ________ (go)to the South Pole(南极)in 1990 for the first time. He ________ ________ ( know ) much more information (信息) about it since then.