You plan to invest $100 per month in an S&P 500 index fund for the next 40 years, and are trying to decide whether to use an ETF or an open ended mutual fund. Which option would be the most advisable? (assume that the ETF and open ended index mutual fund have the same expense ratio, but the broker will charge you a $5 commission for each trade, while the mutual fund will not charge you a commission if purchased directly from the mutual fund)