D because it shows you what tense the verb is relating to the time.
Answer:The specialization of the assembly line meant that Ford no longer had to use craftsmen and could instead hire low-skill workers and teach them a few simple steps. ... Despite the higher labor costs, Ford's efficiencies allowed him to eventually lower the price of the Model T from its introductory rate of $850 to $260.
Probably a survey or a questionnaire on a group of random people so as to sample them. In that way you could cover a large number of people unlike using interviews that are time consuming and can only gather info from a few persons.
