Solution: It was discovered that the shortage of a specific element showed its symptoms first in older leaves and later in younger ones.

a) If a deficiency of a certain element manifested itself first in older leaves and then in younger leaves, this shows that the element is actively mobilized.

b) Nitrogen and Magnesium are two highly mobile elements.

Calcium and Boron are two rather immobile elements.

c) In order to comprehend the deficiency of a certain element, agriculturists and horticulturists noticed that the deficiency of a given element appears first in older leaves and later in younger leaves, indicating that the element is actively mobilized. Since sulfur is an immobile element, its lack manifests itself first in the younger leaves.