“X” is the best reducing agent among the hydrides of 15th group and ‘Y’ is the most volatile 15th group hydride. X & Y respectively are

Reducing power ….    NH3 < PH3 < AsH3 < SbH3< BiH3…….down the group as X - H bond enthalpy decreases , reducing power increasesBoiling point           ….   PH3 < AsH3 < NH3 < SbH3< BiH3Boiling point  α  1volatility