We toss a coin successively. Let Bi be the event that the outcome of the ith toss is Heads. Then, the event “Heads occur for the first time at, or after, the second toss”

can be written, in terms of the sets Bi, as follows:

(a) B1

(b) B1B2

(c) B′

1B2

(d) B′

1B2B3B4 · · ·

(e) B′

1.