问题如下：
Returns to commodity investing are based on:
选项：
A. Changes in price.
B. Dividends.
C. Interests.
解释：
A is correct.
Returns to commodity investing are based on changes in price and do not include an income stream such as dividends, interest, or rent.
这道题我第一眼看到了，想到的是roll yield 就是future and spot price差，这个为什么不对呢？可以解释一下大宗货物交易的收益组成嘛，感谢。
