C8,2 indicates the total number of possible events when 8 flies are released one by one. In other words, choose two of the eight positions for two flies.

2. Analytical molecules:

Molecules represent all the possible events of the remaining k fruit flies after all the flies fly out.

We will record the first flight as 1, the second flight as 2, and so on (easy to describe). Assuming that only K fruit flies are left after two flies fly out, then k- 1 (the last one) must be a fly. At this time, 8-k flies fly out, and the first k- 1 has been determined to be a fly, so it is only necessary to determine the order of the remaining flies, that is, choose a position for another fly in the remaining 8-k- 1 = 7-k, that is, in the form of molecules.

Suppose k=3, that is, there are three fruit flies, then five fruit flies fly out, and the fifth one must be a fruit fly (because all the remaining fruit flies at this time, if the fifth fruit fly is also a fruit fly, then the hole will not be opened and the fruit fly will not be released). Then there is a fly (7-k, k=3) in the first four flies, and the number of possible events = C4, 1 (where 4=7-k, k=3).

I hope it helps you _