Biased/unbiased predictors of spot/future prices

I thought derivatives were easy, until I came across this weird question->

Schweser, Derivatives, page 55, question 10

The question states the following about a specific nation’s market

“…various historical time periods with a high volume of normal backwardation trading, and other periods of time with a high volume of normal contago trading”.

Currently, most of the trading volume evidences normal contago prices

Question - Future prices in this country are most likely:

a. biased predictors of expected spot rates

b. unbiased predictors of exp spot rates

c. lower than expected spot rates

My question: What does contago/backwardation have to do with biased/unabiased?

Answer at the back doesn’t really give any detail about why this is so.

Would appreciate your help in making me feel overconfident again!

bump

My 2 cents:

when it says normal, youre comparing expected spot vs futures.

if its normal contango, id imagine current prices are above expected future spot, therefore A.

what does the Ans at the back say?

http://www.analystforum.com/forums/cfa-forums/cfa-level-ii-forum/91338895

http://www.analystforum.com/forums/cfa-forums/cfa-level-ii-forum/91332227

It was explained by Mr.Magician in the above links.

^ that’s great - thanks!