Options trading strategies

A box spread is a combination of a bull spread composed of two call options with strike prices X1 and X2 and a bear spread composed of two put options with the same two strike prices. All options are European and have the same expiration time.

a). Describe the payoff from a box spread on the expiration date of the options.

b). What would be a fair price for the box spread today? I am not expecting a number, only a precise statement.

c). Under what circumstances might an investor choose to invest in a box spread?