Let R be the reaction of the body to some stimulus of strength s (measured in the appropriate units). The sensitivity S is defined to be the rate of change of the reaction with repect to s. Suppose that, for the particular R and s, the following experimental formula has been used:
R=（90+21s^0.5）/(1+3s^0.5)
The sensitivity S at equals _____ (units of )/(units of )
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