Hkcee 2010 Econ Paper 2 Q2 ((exclusive)) Official

The correct answer is because of how economists define "cost":

“Queuing – buyers will line up early to purchase the good because the price is artificially low, and time becomes the rationing device.” hkcee 2010 econ paper 2 q2

To solve this, one must apply the assumption step-by-step. The correct answer is because of how economists

Similarly, supply: at Q=40, P=$10; Q=60, P=$20; Q=80, P=$30; Q=100, P=$40; Q=120, P=$50. supply: at Q=40