A shop sells hamburgers which contain some of the following: meat burger, vegetable burger, lettuce, tomato, carrot, mayonnaise and tomato sauce. a. You must have a meat burger or a vegetable burger. b. You can have any number of other ingredients or none. c. If you have meat burger, you can also have tomato sauce, but not if you have a vegetable burger. d. If you have lettuce or tomato or both you can have mayonnaise but not otherwise.How many different hamburgers can be constructed according to these rules?

1) 7
2) 21
3) 42
4) 35