An annual ring consists of two kinds of wood that appear as alternate concentric rings. The two types of wood are formed at _A_ time and by the activity of _B_.

Wood is formed due to the activity of vascular cambium.  During spring season,  wood formed is called spring wood or early wood.  In winter, wood formed is known as autumn wood or late wood. The two kinds of woods that appear as alternate concentric rings, constitute an annual ring.