The price of maple syrup has jumped front 22 dollars a gallon three years ago to 40 dollars a gallon today. It can be concluded that maple-syrup harvesters have been artificially inflating prices and that governmental price regulations are necessary to control rising prices.
Which of the following, if true, casts the most doubt on the conclusion drawn above

A．The government already requires maple-syrup harvesters to submit their facilities to licensing by the health department.

B．Insect infestation and drought have stunted the growth of syrup-producing maple trees and caused leas-abundant syrup harvests.

C．Maple syrup is produced in rural areas that suffer from high unemployment.

D．Technological improvements in maple-syrup harvesting have reduced production costs.

E．(E) Maple-syrup prices have risen many times in the past, though never before at the rate recently observed.

罗杰斯提出的学习动机理论是 ( )

A．需要层次论

B．强化理论

C．自我效能感理论

D．自由学习理论