Suqian three-day pressure

A, ignoring the gravity of the glass tube itself, the mercury column gravity and buoyancy are balanced, and the required Zhang Liwei is 0;

B. In this case, the required pulling force should be F=ρ mercury L'Sg, and the size of L 'should be about 760mm instead of1m; ;

C, after the top of the glass tube contacts the mercury surface in the tank, there will be no vacuum at the top, and only when it reaches 760mm or more will there be a vacuum;

D, as long as the space in the tube is greater than 760mm, it will always keep this height unchanged.

So choose D.