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

a)Unlike mutual funds, when you trade ETF shares, you pay commissions.

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

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

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