(2008? Suqian) As shown in the figure, a lightweight glass tube with one end closed, length l (about 1 meter), and cross-sectional area S has negligible wall thickness.

A. When the gravity of the glass tube itself is ignored, the gravity of the mercury column is balanced by the buoyancy force. At this time, the required pulling force is 0;

B. In this case, the required The pulling force should be F=ρmercury L′Sg, and the size of L′ should be about 760mm, not 1m;

C. After the top of the glass tube is exposed to the mercury surface in the tank, there will be no vacuum at the top. It will only appear when it reaches 760mm or above;

D. As long as the space in the tube is greater than 760mm, it will remain at this height.

So choose D.