I only require the answer to the why part.

What is the price of a call option with an exercise price of zero and an infinite time to maturity equal to? The highest price theoretically possible for a call option is to equal the value of the underlying stock. This happens only for a call option that has a zero exercise price and an infinite time until expiration. With such a call, the option can be instantaneously and costless exchanged for the stock at any time. Why