Which of the following statements regarding ETFs (exchange-traded funds) is TRUE?

a) Unlike mutual funds shares, the NAV of ETFs is not always exactly equal to the value of its underlying stock portfolio.

b) Unlike mutual funds, when you trade ETF shares, you pay comissions.

c) Funds that are "open-ended" are referred to in this way because their stock price can potentially rise an infinite amount.

d) Both mutual funds and ETFs are open-end funds.