Which answer best explains how nitrogen deficiency in plants causes chlorosis at the tips of older leaves?

A) Nitrogen is a component of chlorophyll, but it is mobile and is re-located from older leaves to growing parts.
B) Nitrogen is necessary to synthesize chlorophyll in mature areas of older leaves.
C) Nitrogen is a component of all enzymes, so new growth will stop if nitrogen is deficient.
D) Nitrogen regulates proton pumps that supply energy to growing tissue at the tips of older leaves.