单项选择题It follows from the weak axiom of revealed preference that if a consumer chooses x when he could afford y and chooses y when he could afford x； then his income must have changed between the two observations.。
单项选择题《北方的纳努克》受到诸多好评,被认为是世界第一部真正意义上的纪录片,正式开启了纪录片时代。（ ）
