题目

A scientist once said: “I have concluded that the earth is being visited by intelligently controlled vehicles from outer space.”If we take this as a reasonable explanation for UFOs (unidentified flying objects), questions immediately come up.“Why don’t they get in touch with us, then? Why don’t they land right on the White House lawn and declare themselves?” people asked.In reply, scientists say that, while this may be what we want, it may not necessarily be what they want.“The most likely explanation, it seems to me,” said Dr. Mead, “is that they are simply watching what we 如图所示，一支重0.4N的试管管口朝下，管内封住适量的空气，恰好悬浮在水中，此时它受到的浮力是____N。若水温升高，则试管将会___（上浮/悬浮/下沉）。