How to prevent backflow c and control gas flow d in high school chemistry

In C, gas enters from a short pipe and is discharged from a long pipe. If the gas is sucked backwards, the liquid on the right enters the bottle on the left from the long tube instead of flowing out from the short tube, which is called back suction. In D, gas enters from any short pipe and is discharged from the other short pipe. If the gas pressure in the bottle is not enough, there will be backward suction, that is to say, the liquid rises from the middle tube and is not discharged from the short tube, thus preventing backward suction.