The theory of 'use and disuse' was proposed by: MCQ with Answer and Explanation

Explanation:
Gene therapy is an experimental technique that involves introducing genes into a patient's cells to correct or compensate for genetic defects causing disease. Transgenic animals have had their genome deliberately altered by the insertion of foreign DNA from another species. Biopesticides, derived from natural materials like bacteria, plants, or minerals, are generally target-specific and non-toxic to humans and the environment, unlike synthetic chemical pesticides. Thus, statements 1 and 2 are correct.

Explanation:
Scurvy is caused by a deficiency of Vitamin C (ascorbic acid), leading to bleeding gums; rickets is caused by Vitamin D deficiency. Iron is essential for hemoglobin synthesis, and its deficiency leads to iron-deficiency anemia, causing fatigue and weakness. Iodine is required for thyroid hormone production, and its deficiency causes the enlargement of the thyroid gland, known as goiter. Thus, statements 2 and 3 are correct.

Explanation:
Nitrogen fixation is the conversion of inert atmospheric N₂ into ammonia (NH₃) by free-living (e.g., Azotobacter) or symbiotic (e.g., Rhizobium) bacteria, or by industrial processes. Nitrification is the oxidation of ammonia to nitrites then nitrates. Denitrification converts nitrates to N₂ gas. Ammonification is the release of ammonia from organic nitrogen by decomposers. Fixed nitrogen is essential for plant growth.