For each stock in the stock market, the number of shares sold daily equals the number of shares purchased. That is, the quantity of each firm's shares demanded equals the quantity supplied. So, if this equality always occurs, why do the prices of stock shares ever change? Please select the correct answer from choices A,B,C or D!

a.Prices are set at a different level each day by Wall Street traders.

b.Prices seldom change.

c.Prices change due to the whims of those selling shares.

d.Prices change in reaction to a mismatch between Qd and Qs.