if wxy<0, xyz>0, is z>0?
St1... (-)(-)=(+) and (-)(-)(-) = (-) and since wxy is negative so if w<0 then
either x and y are both +ve
or x and y are both -ve
but it doesn't give further information. St1 insuff.
St2... says that x is +ve and that suggest that
either both y and z are positive
or both y and z are negative
no further information. St2 insuff.
Combined: by combining both we knows that both x and y are positive and since xyz is positive then z has to be positive. Suff.
IMO answer is C