Missing A and B in the circuit

Well, I'm not a professional, just interested in circuit diagrams. A few days ago, when I was looking for some information about the gas sensors, I saw the circuit diagram in the article and I couldn't understand it.

It says that it's a gas sensor module. But there's no A and B in the picture.

If you guys are interested, the circuit of the gas sensor is in here. Can someone help me explain that?


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