When watching an ETF (e.g., VTI) via yahoo finance during the day - the price is changing. Is it because

A)the holdings of the fund change in value OR
B)demand/offering play a role in the price.

The B option seems very odd to me. Is a price of an ETF affected by market demand? (on Thursday 10am vs. Thursday 10:15am?

Kay answered:
ETFs (Exchange Traded Funds) fluctuate in price through out the trading period of stocks since ETFs are mutual funds. Your answers A and B are both correct. Generally, market price is determined by supply and demand. More people who wants to buy a certain ETF brings its price higher since more buyers than sellers. At the same time, stock holdings in the fund also affect the value of the fund but not necessarily a daily market price.