The Primary Difference Between an Index Fund and an Exchange

The primary difference between an index fund and an exchange traded fund (ETF) is the

A) Higher expenses associated with the ETF
B) Fact that you can short sell an index fund, but not an ETF
C) Fact that you pay commission to purchase an index fund, but not an ETF
D) Greater variety of industry-specific offerings provided by index funds as compared to ETFs
E) Fact that ETFs are traded like stocks throughout the trading period