Introduction to Commodities and commodity derivatives

Reading 42 EOC Q14 from the material provided by CFAI

why is the index that contains commodities in backwardation able to generate a better return than the index containing contango commodities when the market is trending upward. Wont the contango index generate a positive roll return and price yield when the market is upward trending ?

Both indexes use similar weighting and rebalancing schemes.

Can someone answer this question?