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 1/6 taller than it was at the end of the 4th year. By how many feet did the height of the tree increase each year?	Pick:
A) 3 / 10 B) 2 / 5 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 / 6 ) ( 4 + 4 x ) or , x = 1 / 2 answer c"
Answer: C