Partial pressures

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B is the correct answer. I posted it from my phone and just looked at my laptop and realized the awful quality. Sorry! Can someone explain why the partial pressure of O2 and N2 would decrease? Thanks!
 
I think the total pressure will be constant. As the moles of water vapor increase, the denominator in the equation for mole fraction increases. Therefore the mole fractions of N2 and O2 are decreased. Since partial pressure = mole fraction x total pressure, the partial pressures of N2 and O2 must decrease.
 
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