Consider the following statements regarding mineral nutrition:
$(a)$ Moderate decrease or increase of micronutrients causes deficiency and toxicity symptoms respectively.
$(b)$ Excess of manganese causes toxicity of iron, calcium and molybdenum.
$(c)$ $A$ macronutrient is said to be toxic when present below a critical concentration.
Which of the above statements are correct?

• A
  Only $(a)$ is correct
• B
  Only $(b)$ is correct
• C
  $(b)$ and $(c)$ are correct
• D
  $(a)$ and $(c)$ are correct