Natural monopoly occurs when

(a) long-run average costs decline continuously through the range of demand.
(b) economies of scale are obtained at relatively low levels of output.
(c) a firm owns or controls some resource essential to production.
(d) long-run average costs rise continuously as output is increased.

Aproperty title search firm is contemplating using online software to increase its search productivity. currently an average of 40 minutes is needed to do a title search. the researcher cost is $2 per minute. clients are charged a fee of $400. company a's software would reduce the average search time by 10 minutes, at a cost of $3.50 per search. company b's software would reduce the average search time by 12 minutes at a cost of $3.60 per search. which option would have the higher productivity in terms of revenue per dollar of input?