Assertion (A): Double circulation is seen in birds and mammals. Reason (R): It ensures complete separation of oxygenated and deoxygenated blood. MCQ with Answer and Explanation

Explanation:
Vitamin K is essential for the synthesis of clotting factors (II, VII, IX, X) in the liver. Deficiency leads to prolonged clotting time and excessive bleeding. Vitamin A is for vision, C for collagen (scurvy causes bleeding gums but not clotting defect), D for bones. Newborns receive vitamin K injection to prevent hemorrhagic disease. Green leafy vegetables are rich sources.

Explanation:
Collenchyma cells have unevenly thickened walls, providing mechanical support and flexibility to growing plant parts like stems. Sclerenchyma consists of dead cells with heavily lignified, thick walls, providing rigid strength to plant parts. Parenchyma cells are living, thin-walled, and serve as the primary packing tissue, storing food, water, and air. All three statements are correct.

Explanation:
Prophase I of meiosis is divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis. Crossing over, the exchange of genetic material between non-sister chromatids of homologous chromosomes, occurs specifically during the pachytene stage at structures called recombination nodules.