Answer is: a secretary smokes a cigarette in a crowded break room.
External cost Is when consuming a good or servise imposes a cost to a third party. In this case, smoking affects other people in break room, causing pollution and health related problems.
Directional selection I'm pretty sure hope it helps!
Both organs are made up if several kinds of tissues. :D
If you conduct experiments and get consistent results and then change variables and again get consisnet results.