Annual Growth Rings in Trees Are the Result of What

Annual growth rings in trees are the result of what?

A) the cyclical growth and death of ground tissues in the phloem
B) the seasonal variation in vascular cambium, consisting of the development of thin cell walls in secondary xylem in the spring and thicker cells walls in secondary xylem in the fall.
C) the annual variation in rapid growth in the apical meristems that results in the thickening of the cork cambium layers.