Which part of the flower is responsible for producing pollen grains? MCQ with Answer and Explanation

Explanation:
The blood-brain barrier (BBB) is formed by tight junctions between endothelial cells of brain capillaries, restricting the passage of pathogens and large molecules. Meninges cover the brain, choroid plexus makes CSF, corpus callosum connects hemispheres.

Explanation:
Natural selection is the process where organisms with traits better suited to their environment are more likely to survive and reproduce, leading to adaptation of the population over time. Mutations, which are random changes in the DNA sequence, are the ultimate source of all new genetic variation upon which evolutionary forces act. Evolution is a population-level phenomenon, defined as a change in allele frequencies over generations; it does not occur within a single individual's lifetime. Thus, statements 1 and 2 are correct.

Explanation:
Vitamin D (specifically cholecalciferol, Vitamin D3) is unique because it can be synthesized endogenously in the human skin. When 7-dehydrocholesterol in the skin is exposed to ultraviolet B (UVB) radiation from sunlight, it is converted to previtamin D3, which then isomerizes to Vitamin D3. It is subsequently hydroxylated in the liver and kidneys to its active hormonal form, calcitriol, which regulates calcium and phosphorus metabolism.