By definition, a down-and-out call option on stock becomes worthless and terminates if the price of the

Question:

By definition, a down-and-out call option on stock becomes worthless and terminates if the price of the underlying stock moves down and touches a pre-specified point during the life of the call. If the pre-specified level is $75, for example, the call expires worthless if and when the stock price falls to $75. Describe, without a diagram, how a binomial tree can be used to value a down-and-out call option.