Again, suppose the first bill that is introduced mandates that security be improved so that the probability of catching a terrorist at the border increases from $10\%$ to $20\%,$ and these measures do not change the position of the blue curve. The opportunity cost of this increase in security is million visitors per year.

Suppose the first bill is passed, raising the probability of catching any given terrorist from $10\%$ to $20\%.$ However, this isn't enough for some lawmakers. One representative introduces a bill that would increase security by an additional $10$ percentage points.

Again, assume these measures do not change the position of the blue curve. The opportunity cost of this additional measure is million additional visitors per year.

Refer back to your previous answers. The opportunity cost of increasing the probability of catching a terrorist from $20\%$ to $30\%$ is the opportunity cost of increasing that probability from $10\%$ to $20\%.$