From the following list, single out those measurements for which the square root of a typical measurement is a proper estimate of the standard deviation of the distribution from which the measurement is drawn:
(a) A 1-min count from a detector.
(b) A 5-min count from a detector.
(c) The net counts from a detector over a 1-min period after background subtraction.
(d) The counting rate expressed as counts per second based on a 100-s measurement.
(e) The average of five sequential 1-min counts.
(f) The sum of five sequential 1-min counts.