In a city, the marginal benefit of an urban public transport mode is given by 20-2q, where q is the number of passengers. The total cost of this transport mode is given by 2q+40.

Considering this information, please answer the following questions:

a) What is the efficient number of passengers per day?

b) What is the budget-balancing allocation for this transport mode?

c) If it is necessary, how much should be the subsidy to have an efficient number of passengers per day?

d) Considering that authorities are not able to assess accurately the costs of this monopoly, what will happen to this transport mode costs over time? And how might it affect passengers in the future?