A total of 12 cells are replicated. Freshly synthesized DNA cannot be replicated again until mitosis is completed. Two control mechanisms have been identified-one positive and one negative-that are used with equal probability. Assume that each cell independently uses a control mecha- nism. Determine the following probabilities.

(a) All cells use a positive control mechanism.

(b) Exactly half the cells use a positive control mechanism.

(c) More than four, but less than seven, cells use a positive control mechanism.