20 13 On April 20th, an earthquake of magnitude 7.0 occurred in Lushan County, Ya 'an City, Sichuan Province. After the earthquake, the party and the government promptly organized personnel to carry o

20 13 On April 20th, an earthquake of magnitude 7.0 occurred in Lushan County, Ya 'an City, Sichuan Province. After the earthquake, the party and the government promptly organized personnel to carry out scientific rescue. A, the blade is very sharp, which is to reduce the stress area to increase the pressure on the steel bar when the pressure is constant, so A is correct;

B, because the air-dropped materials fall at a constant speed, the force it receives is a balancing force, so B is correct;

C. in the phenomenon that the syringe sucks out the liquid medicine, the liquid medicine is not sucked up, but is pressed up by atmospheric pressure, so c is wrong;

D, the doctor used B-ultrasound to check whether the wounded internal organs were damaged, but used ultrasound, so D was wrong.

So choose ab.