Please make sure it is the correct answer(s) and with explanation, thank you)

Why must real options have positive value? (Select all the choices that apply.) A. If exercising the real option would reduce value, managers can allow the option to go unexercised. B. Having the real option but not the obligation to act is valuable. C. Real options must have positive value because they are only exercised when doing so would increase the value of the investment. D. Real options must have positive value because they can always be sold to recover the initial investment