4. Three coins are flipped. The sample space representing all the outcomes of this action is the set S={TTT, TTH, THT, THH, HTT, HTH, HHT, HHH}.
a. List the subset of represents the event "At least one tail is flipped". __________________________________
b. List the subset of represents the event "At least two tail is flipped".__________________________________
c. List the subset of represents the event "At least one head is flipped".________________________________
d. List the subset of represents the event "At least two head is flipped".________________________________