A gardener plants trees in an arithmetic pattern. She plants a certain number of trees on the first day and increases the number by the same amount each subsequent day. On the 7th day she plants 34 trees, and the total number of trees planted in the first 7 days is 133. What is the common difference in the number of trees planted per day?
(a) 6
(b) 5
(c) 4
(d) 7