Answer:
This is true, the efficient market hypothesis only holds if all the investors are rational, for example if an investor is not rational and wants to make a loss instead of profit, then the efficient market hypothesis wont hold as the investor will be acting in a way that wont benefit him. When the investor acts irrationally, then he wont react correctly to the information he has and buy or sell stocks which he isn't supposed to buy or sell and this will change the price of the stock from what the price of the stock should be.
Explanation:
