Two cold tablets are unintentionally placed in a box containing two aspirin tablets. The four tablets are identical in appearance. One tablet is selected at random from the box and is swallowed by the first patient. A tablet is then selected at random from the three remaining tablets and is swallowed by the second patient. Define the following events as specific collections of simple events:
a. The sample space S
b. The event A that the first patient obtained a cold tablet
c. The event B that exactly one of the two patients obtained a cold tablet
d. The event C that neither patient obtained a cold tablet