问题如下：
Which of the following statements regarding exchange-traded funds (ETFs) is correct? ETFs:
选项：
A. disclose their holdings on a quarterly basis
B. trade in both primary and secondary markets.
C. offer a creation/redemption mechanism that allows any investor to create or redeem shares.
解释：
B is correct. ETFs trade in both primary and secondary markets. The primary market for ETF trading is that which exists on an over-the-counter basis between authorized participants (APs), a special group of institutional investors, and the ETF issuer or sponsor. This process is referred to as creation/redemption, and it is only through these primary market transactions that shares of the ETF can be created or destroyed. ETFs also trade in the secondary market on exchanges. Secondary market trading happens between any pair of market participants—individual or institutional investors, market makers, and so on.
