问题如下：
Investors buying ETFs:
选项：
A. incur management fees that decrease with the length of the holding period.
B. are assured of paying a price equal to the NAV if they purchase shares at the market close.
C. incur trading costs in the form of commissions and bid–ask spreads at the time of purchase
解释：
C is correct.
ETF trading costs in the form of commissions and bid–ask spreads are paid by investors buying or selling ETF shares on an exchange. These trading costs are influenced by the bid–ask spread of the ETF, the size of the trade relative to the normal trading activity of the ETF, and the ease of hedging the ETF by the market-making community. Even the closing price of the ETF on the exchange includes a premium or discount to the NAV, driven by supply and demand factors on the exchange and the market impact costs of executing an exchange transaction. The purchase and sale trading costs of an ETF are paid regardless of holding period, whereas other costs, such as management fees, increase as the holding period lengthens.
老师，讲下A选项。谢谢
