In the Oral Approach, Palmer insisted that there was a basic difference between the spontaneous capacities of human beings to ______ language naturally and unconsciously and the trained or"situational" capacities of the classroom learner which allowed him to learn language consciously.

当消费者收入提高时，下述哪条曲线必然向右上方移动（）

A.预算线

B.无差异曲线

C.成本曲线

D.供给曲线