The initial margin for an S&P 500 contract is 15% of the notional value, while the initial margin for a 10 year Treasury bond contract of similar notional size is 2%. What accounts for the difference in initial margin?

 a. The S&P 500 Index is more volatile than the 10 yr. T-Bill.

b. The S&P 500 Index has a higher notional value than the 10 yr. T-bill.

c. The contract size of the S&P 500 Index is larger than the 10 yr. T-bill. 

d. The price of the S&P 500 Index is much larger than the 10 yr. T-bill

e. This is a mispricing that can be arbitraged away