12. A cell has two types of ion channels, type A and type B. The cell has NA=4 indistinguishable channels of type A which are each independently open with probability pA=0.2, and the cell has NB=5 indistinguishable channels of type B which are each independently open with probability p=0.1. a) What is the probability that the cell has no channels open? b) What is the probability that the cell has exactly one channel open (of either type)? c) What is the probability that the cell has at least one channel of type A open, and at least one channel of type B open?