问题如下：
An analyst thinks that the price of A crude oil will increase more than that of B crude oil. Based on this, which position should the analyst take?
选项：
A.a basis swap long on A crude oil and short on B crude oil
B.a basis swap long on B crude oil and short on A crude oil
C.a short position in an excess return swap that is based on a fixed level of the commodity Index.
解释：
A is correct.
basis swap基于两种类似商品的价格变动，分析师预测A原油的价格增长会高于B原油的价格增长，所以应该是Long A, short B. 同时，题目并没有给出与index的相对变化，所以不能用excess return swap，而是要用basis swap。
excess return swap 怎么用啊？什么时候用啊？
