...the mugs and the chocolate cost about the same, and in pretests students. were as likely to choose one as the other.
yet when offered the opportunity to switch from a mug to a chocolate bar or vice versa, only one in the switched.

이 문장들 해석 좀 부탁드립니다. 너무 매끄럽게 되지 않아서요 .ㅜㅠㅜㅠ
친절한 답변 부탁드립니다 .^^