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A researcher in physiology has decided that a good mathematical model for the number of impulses fired after a nerve has been stimulated is given by y= - x" +30x -70, where y is the number of responses per millisecond and x is the number of milliseconds since the nerve was stimulated. (a) When will the maximum firing rate be reached? (b) What is the maximum firing rate? (a) The maximum number of impulses fired occurs at milliseconds. (b) The maximum number of impulses per millisecond is