The gamma for a portfolio of options on the same stock is: (Show your reasonings)

1. the sum of the gamma for the individual options divided by the number of units of each option.
2. cannot be computed if the options have different strikes.
3. the sum of the gamma for the individual options times one minus the percentage of each option in the portfolio's value.
4. the sum of the gamma for the individual options times the percentage of each option in the portfolio's value.
5. the sum of the gamma for the individual options times the number of units of each option.