We throw a die until a six appears for the first time, at which point the experiment stops. Let Ai be the event that the outcome of the ith throw is a six. The event “a six appears for the first time in the 3rd throw” can be expressed in terms of the sets Ai as

(a) A1A2A3

(b) A1A2A′

3

(c) A′

1A′

2A3

(d) A1 ∪ A2 ∪ A3

(e) A1 ∪ A2 ∪ A′

3.