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here is a quick counting question:

A counter near a long-lived radioactive source measures an average of 100 counts per minute. The probabilty that more than 110 counts will be recorded in a given one-minute interval is most nearly

(A) zero

(B) .001

(C) .025

(D) .15

(E) .5

I kinda guess that it is D, .15, but I am not able to explain it accurately, other than it is within one standard deviation.