I just took a quiz on this! It's D!
Moles of 
,
Moles of acetic acid 
,
Mole fraction of water :
Therefore, mole fraction of water in this solution is 0.645 .
Hence, this is the required solution.
He valence of selenium depends on which compound it is in. Selenium is very similar to sulfur. It may have a valence of 6; example selenium hexafluoride SeF6, selenium trioxide SeO3
<span>4; example selenium tetrafluoride SeF4, selenium dioxide SeO2 </span>
<span>2; example selenium difluoride SeF2, selenium dichloride SeCl2 </span>
<span>and -2; example hydrogen selenide H2Se</span>
