Find the incorrect statement.

• A
  Gene therapy is a genetic engineering technique used to treat diseases at the molecular level by replacing defective genes with normal genes.
• B
  Calcitonin is a medically useful recombinant product in the treatment of infertility.
• C
  Bt toxin is a biodegradable insecticide obtained from $Bacillus \text{ } thuringiensis$.
• D
  $Trichoderma$ species is a biocontrol agent for fungal diseases of plants.