Vent pipe placement
I'm running the pipes for a second floor bathroom in a new addition. I have the lav and shower draining to an exterior gray water area and the water closet with it's own 3" pipe going to the existing septic tank. I'm not that concerned where the vent for the wc goes, since it's on it's own drain. I was hoping to get away with only one vent for the lav/shower and have attached a picture of the layout. I wondered if I could go with just vent A or just B or if I needed both. Any other comments are welcome.