"But this approach must be treated with great caution. The number of cases in which it might be justified will always be small. It does not give a green light—to the United States or any other nation—to act first without exhausting other means, including diplomacy. Preemptive action does not come at the beginning of a long chain of effort. The threat must be very grave. And the risks of waiting must far outweigh the risks of action." Which sentence of the paragraph above is the main idea?

A: But this approach must be treated with great caution.
B: Preemptive action does not come at the beginning of a long chain of effort.
C: The number of cases in which it might be justified will always be small. It does not give a green light—to the United States or any other nation—to act first without exhausting other means, including diplomacy.
D: And the risks of waiting far outweigh the risk of action.