Help with question

The passage explains that gigantic passenger planes are being developed that will have enough space to accommodate shops and lounges in addition to seating for passengers. The argument then adds, however, that this additional space, though could hold these shops and lounges, will most likely be used for more passenger seating. Why? Because the number of passengers flying the air-traffic system will most likely triple in the next 20 years, making it impossible for airports to accommodate enough normal-sized jet planes to carry that many passengers. Thus, the last sentence, i.e. answer choice (D) is supporting the conclusion that this additional space will probably be used for more passenger seating. Remember that the conclusion of an argument will never support any other sentence in the argument. The main point of the argument is: though the additional space could fit shops and lounges, it will most likely be used for more passenger seating, i.e. answer choice (B), since within the next 20 years the number of passengers flying the air-traffic system is expected to triple, which will make it impossible for airports to accommodate enough normal-sized jet planes to carry that many passengers. Hope that clears things up! Please let us know if you have any other questions.