Explain why a characteristic of an efficient market is that investments in that market have zero NPVs?

On average, the only return that is earned is the

required return-investors buy assets with returns in

excess of the required return (positive NPV), bidding

up the price and thus causing the return to fall to the

required return (zero NPV); investors sell assets with

returns less than the required return (negative NPV),

driving the price lower and thus the causing the

return to rise to the required return (zero NPV).