Fire projection density refers to the maximum amount of ammunition fired by the military sector in unit time during military operations. It is one of the important indicators for measuring the combat effectiveness of combat troops in modern military science. According to the above definition, which of the following is the following to enhance the combat effectiveness of the troops by increasing the fire projection density? ["An infantry division selects special shooters among ordinary soldiers and arranges them into a sniper squad, which can more effectively kill the enemy's living force.", 'A country replaced all semi-automatic rifles in its queue with fully automatic rifles, increasing the firing frequency of bullets.', 'An artillery unit extended the originally planned two-hour gunfire cover time to three hours, which more effectively killed the enemy in the bunker.', 'A military scientific research unit has developed a more advanced gunfire positioning system, which can reduce the fire strike error from a radius of 200 meters to 50 meters.']