NT Econ fam can you explain this?

So the question was "Supposed Darnell is using combination D, producing 1 boat. What is his opportunity cost of producing a second boat?"

The answer is 5 Kites and I dont know how to get that answer.

This isn't a homework btw. Its a practice worksheet for students to prepare for the test.

Theres only 8 hours that can be used.

so in combination D, he makes 1 boat and 24 kites.

1 boat = 2 hour

24 kites = 6 hour.

24/6 = 4

so 1 hour =4 kites, right?

so 2 boat = 4 hours

which means 4 hour = 16 kites.

Giving up 2 hours means he gave up 8 kites so that should be his opportunity cost, right?

my brain hurts.

If you look at choice C and D, having one fewer boat is the opportunity cost of 5 kites produced because in choice C 2 boats produce 19 kites, and if he chooses D then one fewer boat produces 24 kites or 5 extra kites