I'd say D.
r^2 is always > 0
Similarly t^4 is always > 0
Hence if s is positive then, r^2*s^3*t^-4>0
From (1) - s is greater than square of a number - so s > 0
From (2) - s > 1/t^4 - since t^4 > 0, s > 0
Hence either statement gives enough information to say that the expression will be > 0
So answer should be D.
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