When a certain tree was first planted, it was 4 feet tall, and the height of the tree increased by a constant amount each year for the next 6 years. At the end of the 6th year, the tree was 1/3 taller than it was at the end of the 4th year. By how many feet did the height of the tree increase each year?
Choose one.
A) 3 / 10	B) 2	C) 1 / 2	D) 2 / 3	E) 6 / 5

Rationale: "say , the tree grows by x feet every year . then , 4 + 6 x = ( 1 + 1 / 3 ) ( 4 + 4 x ) or , x = 2 answer b"
Answer: B