In this experiment, we are not noting the sequence of heads and tails that come up. (If we were, then choice B would be the correct answer.) What we are doing is counting the number of times heads comes up. The possibilities are:

0 (no heads come up)
1 (heads comes up once)
2 (heads comes up twice)
3 (heads comes up three times)

Therefore, we can write the sample space as {0, 1, 2, 3}.

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