Each of the following pairs represents a likely sugar source and its sink except _________.

A. An apple seed and the fruit in which it is contained;

B. Mature leaves of a tomato plant and a developing tomato;

C. Newly emerged leaves on a bean seedling and the seedling’s developing roots;

D. A sugar beet root in spring and its developing flower stalk;

E. The roots of an oak tree and its leaf buds in spring.