Doesn't it have to be "water main"? Or are the two interchangeable?
M：Then how about staying at my younger sisterís apartment? Itís near my apartment. She has gone on
winter vacation. If you come here, Iíll give you her apartment key.
W：Thank you so much, but shouldnít I ask for her permission?
M：Itís okay. She wonít care since she knows you so well. Oh, donít forget to turn off the main water when you leave.
W：Okay. Youíre my savior.
Student or Learner