How to get to the third level of the death center?

There are two roads in single mode, one is commonly used: (Note: it is better to have soil or bile bottles) After coming out of the safe house, turn right and find the elevator to the second floor. When you get to the iron sliding door, find the elevator to the first floor, go straight for the gun, then turn right to find the elevator to the second floor, find a white inclined door to the second floor, enter (you can guard the door for melee and supply), go straight and turn right, and you will see a door going downstairs, and then turn right. After opening the second door, the alarm sounded. At this time, there is an elevator on the right. Go upstairs (don't pull too far with your teammates) and turn left. Go straight to the elevator and turn left at the third floor. Then throw a pottery or bile bottle on the first floor (attracting infinite corpse tide) and wait for 4-8 seconds. Bile can wait a little longer before going in and turning off the alarm. Then continue to walk to another door, turn right to the warehouse and sweep the zombies with a gun for a while, then continue to turn left to find the elevator and slowly find the security room on the second floor (basically no threat)! Raise your hands, remember to give the best.